I believe the question "which is slower" depends on more than just the preamble. As you noted, it may make sense to postpone speed penalties until they are actually used.
This answer is just a partial answer within my scope and interest, so you may need further input regarding the "factor 6 speed penalty" for your example preamble.
In the following example, luatex is three times faster. It is the answer of @percusse posted at pgfplots: Color a 3D surf plot based on y-value (the only modification is that I added a missing round brace in the math expression):
\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12,
colormap/viridis/.style={
colormap={viridis}{
rgb=(0.26700401, 0.00487433, 0.32941519)
rgb=(0.26851048, 0.00960483, 0.33542652)
rgb=(0.26994384, 0.01462494, 0.34137895)
rgb=(0.27130489, 0.01994186, 0.34726862)
rgb=(0.27259384, 0.02556309, 0.35309303)
rgb=(0.27380934, 0.03149748, 0.35885256)
rgb=(0.27495242, 0.03775181, 0.36454323)
rgb=(0.27602238, 0.04416723, 0.37016418)
rgb=(0.2770184 , 0.05034437, 0.37571452)
rgb=(0.27794143, 0.05632444, 0.38119074)
rgb=(0.27879067, 0.06214536, 0.38659204)
rgb=(0.2795655 , 0.06783587, 0.39191723)
rgb=(0.28026658, 0.07341724, 0.39716349)
rgb=(0.28089358, 0.07890703, 0.40232944)
rgb=(0.28144581, 0.0843197 , 0.40741404)
rgb=(0.28192358, 0.08966622, 0.41241521)
rgb=(0.28232739, 0.09495545, 0.41733086)
rgb=(0.28265633, 0.10019576, 0.42216032)
rgb=(0.28291049, 0.10539345, 0.42690202)
rgb=(0.28309095, 0.11055307, 0.43155375)
rgb=(0.28319704, 0.11567966, 0.43611482)
rgb=(0.28322882, 0.12077701, 0.44058404)
rgb=(0.28318684, 0.12584799, 0.44496 )
rgb=(0.283072 , 0.13089477, 0.44924127)
rgb=(0.28288389, 0.13592005, 0.45342734)
rgb=(0.28262297, 0.14092556, 0.45751726)
rgb=(0.28229037, 0.14591233, 0.46150995)
rgb=(0.28188676, 0.15088147, 0.46540474)
rgb=(0.28141228, 0.15583425, 0.46920128)
rgb=(0.28086773, 0.16077132, 0.47289909)
rgb=(0.28025468, 0.16569272, 0.47649762)
rgb=(0.27957399, 0.17059884, 0.47999675)
rgb=(0.27882618, 0.1754902 , 0.48339654)
rgb=(0.27801236, 0.18036684, 0.48669702)
rgb=(0.27713437, 0.18522836, 0.48989831)
rgb=(0.27619376, 0.19007447, 0.49300074)
rgb=(0.27519116, 0.1949054 , 0.49600488)
rgb=(0.27412802, 0.19972086, 0.49891131)
rgb=(0.27300596, 0.20452049, 0.50172076)
rgb=(0.27182812, 0.20930306, 0.50443413)
rgb=(0.27059473, 0.21406899, 0.50705243)
rgb=(0.26930756, 0.21881782, 0.50957678)
rgb=(0.26796846, 0.22354911, 0.5120084 )
rgb=(0.26657984, 0.2282621 , 0.5143487 )
rgb=(0.2651445 , 0.23295593, 0.5165993 )
rgb=(0.2636632 , 0.23763078, 0.51876163)
rgb=(0.26213801, 0.24228619, 0.52083736)
rgb=(0.26057103, 0.2469217 , 0.52282822)
rgb=(0.25896451, 0.25153685, 0.52473609)
rgb=(0.25732244, 0.2561304 , 0.52656332)
rgb=(0.25564519, 0.26070284, 0.52831152)
rgb=(0.25393498, 0.26525384, 0.52998273)
rgb=(0.25219404, 0.26978306, 0.53157905)
rgb=(0.25042462, 0.27429024, 0.53310261)
rgb=(0.24862899, 0.27877509, 0.53455561)
rgb=(0.2468114 , 0.28323662, 0.53594093)
rgb=(0.24497208, 0.28767547, 0.53726018)
rgb=(0.24311324, 0.29209154, 0.53851561)
rgb=(0.24123708, 0.29648471, 0.53970946)
rgb=(0.23934575, 0.30085494, 0.54084398)
rgb=(0.23744138, 0.30520222, 0.5419214 )
rgb=(0.23552606, 0.30952657, 0.54294396)
rgb=(0.23360277, 0.31382773, 0.54391424)
rgb=(0.2316735 , 0.3181058 , 0.54483444)
rgb=(0.22973926, 0.32236127, 0.54570633)
rgb=(0.22780192, 0.32659432, 0.546532 )
rgb=(0.2258633 , 0.33080515, 0.54731353)
rgb=(0.22392515, 0.334994 , 0.54805291)
rgb=(0.22198915, 0.33916114, 0.54875211)
rgb=(0.22005691, 0.34330688, 0.54941304)
rgb=(0.21812995, 0.34743154, 0.55003755)
rgb=(0.21620971, 0.35153548, 0.55062743)
rgb=(0.21429757, 0.35561907, 0.5511844 )
rgb=(0.21239477, 0.35968273, 0.55171011)
rgb=(0.2105031 , 0.36372671, 0.55220646)
rgb=(0.20862342, 0.36775151, 0.55267486)
rgb=(0.20675628, 0.37175775, 0.55311653)
rgb=(0.20490257, 0.37574589, 0.55353282)
rgb=(0.20306309, 0.37971644, 0.55392505)
rgb=(0.20123854, 0.38366989, 0.55429441)
rgb=(0.1994295 , 0.38760678, 0.55464205)
rgb=(0.1976365 , 0.39152762, 0.55496905)
rgb=(0.19585993, 0.39543297, 0.55527637)
rgb=(0.19410009, 0.39932336, 0.55556494)
rgb=(0.19235719, 0.40319934, 0.55583559)
rgb=(0.19063135, 0.40706148, 0.55608907)
rgb=(0.18892259, 0.41091033, 0.55632606)
rgb=(0.18723083, 0.41474645, 0.55654717)
rgb=(0.18555593, 0.4185704 , 0.55675292)
rgb=(0.18389763, 0.42238275, 0.55694377)
rgb=(0.18225561, 0.42618405, 0.5571201 )
rgb=(0.18062949, 0.42997486, 0.55728221)
rgb=(0.17901879, 0.43375572, 0.55743035)
rgb=(0.17742298, 0.4375272 , 0.55756466)
rgb=(0.17584148, 0.44128981, 0.55768526)
rgb=(0.17427363, 0.4450441 , 0.55779216)
rgb=(0.17271876, 0.4487906 , 0.55788532)
rgb=(0.17117615, 0.4525298 , 0.55796464)
rgb=(0.16964573, 0.45626209, 0.55803034)
rgb=(0.16812641, 0.45998802, 0.55808199)
rgb=(0.1666171 , 0.46370813, 0.55811913)
rgb=(0.16511703, 0.4674229 , 0.55814141)
rgb=(0.16362543, 0.47113278, 0.55814842)
rgb=(0.16214155, 0.47483821, 0.55813967)
rgb=(0.16066467, 0.47853961, 0.55811466)
rgb=(0.15919413, 0.4822374 , 0.5580728 )
rgb=(0.15772933, 0.48593197, 0.55801347)
rgb=(0.15626973, 0.4896237 , 0.557936 )
rgb=(0.15481488, 0.49331293, 0.55783967)
rgb=(0.15336445, 0.49700003, 0.55772371)
rgb=(0.1519182 , 0.50068529, 0.55758733)
rgb=(0.15047605, 0.50436904, 0.55742968)
rgb=(0.14903918, 0.50805136, 0.5572505 )
rgb=(0.14760731, 0.51173263, 0.55704861)
rgb=(0.14618026, 0.51541316, 0.55682271)
rgb=(0.14475863, 0.51909319, 0.55657181)
rgb=(0.14334327, 0.52277292, 0.55629491)
rgb=(0.14193527, 0.52645254, 0.55599097)
rgb=(0.14053599, 0.53013219, 0.55565893)
rgb=(0.13914708, 0.53381201, 0.55529773)
rgb=(0.13777048, 0.53749213, 0.55490625)
rgb=(0.1364085 , 0.54117264, 0.55448339)
rgb=(0.13506561, 0.54485335, 0.55402906)
rgb=(0.13374299, 0.54853458, 0.55354108)
rgb=(0.13244401, 0.55221637, 0.55301828)
rgb=(0.13117249, 0.55589872, 0.55245948)
rgb=(0.1299327 , 0.55958162, 0.55186354)
rgb=(0.12872938, 0.56326503, 0.55122927)
rgb=(0.12756771, 0.56694891, 0.55055551)
rgb=(0.12645338, 0.57063316, 0.5498411 )
rgb=(0.12539383, 0.57431754, 0.54908564)
rgb=(0.12439474, 0.57800205, 0.5482874 )
rgb=(0.12346281, 0.58168661, 0.54744498)
rgb=(0.12260562, 0.58537105, 0.54655722)
rgb=(0.12183122, 0.58905521, 0.54562298)
rgb=(0.12114807, 0.59273889, 0.54464114)
rgb=(0.12056501, 0.59642187, 0.54361058)
rgb=(0.12009154, 0.60010387, 0.54253043)
rgb=(0.11973756, 0.60378459, 0.54139999)
rgb=(0.11951163, 0.60746388, 0.54021751)
rgb=(0.11942341, 0.61114146, 0.53898192)
rgb=(0.11948255, 0.61481702, 0.53769219)
rgb=(0.11969858, 0.61849025, 0.53634733)
rgb=(0.12008079, 0.62216081, 0.53494633)
rgb=(0.12063824, 0.62582833, 0.53348834)
rgb=(0.12137972, 0.62949242, 0.53197275)
rgb=(0.12231244, 0.63315277, 0.53039808)
rgb=(0.12344358, 0.63680899, 0.52876343)
rgb=(0.12477953, 0.64046069, 0.52706792)
rgb=(0.12632581, 0.64410744, 0.52531069)
rgb=(0.12808703, 0.64774881, 0.52349092)
rgb=(0.13006688, 0.65138436, 0.52160791)
rgb=(0.13226797, 0.65501363, 0.51966086)
rgb=(0.13469183, 0.65863619, 0.5176488 )
rgb=(0.13733921, 0.66225157, 0.51557101)
rgb=(0.14020991, 0.66585927, 0.5134268 )
rgb=(0.14330291, 0.66945881, 0.51121549)
rgb=(0.1466164 , 0.67304968, 0.50893644)
rgb=(0.15014782, 0.67663139, 0.5065889 )
rgb=(0.15389405, 0.68020343, 0.50417217)
rgb=(0.15785146, 0.68376525, 0.50168574)
rgb=(0.16201598, 0.68731632, 0.49912906)
rgb=(0.1663832 , 0.69085611, 0.49650163)
rgb=(0.1709484 , 0.69438405, 0.49380294)
rgb=(0.17570671, 0.6978996 , 0.49103252)
rgb=(0.18065314, 0.70140222, 0.48818938)
rgb=(0.18578266, 0.70489133, 0.48527326)
rgb=(0.19109018, 0.70836635, 0.48228395)
rgb=(0.19657063, 0.71182668, 0.47922108)
rgb=(0.20221902, 0.71527175, 0.47608431)
rgb=(0.20803045, 0.71870095, 0.4728733 )
rgb=(0.21400015, 0.72211371, 0.46958774)
rgb=(0.22012381, 0.72550945, 0.46622638)
rgb=(0.2263969 , 0.72888753, 0.46278934)
rgb=(0.23281498, 0.73224735, 0.45927675)
rgb=(0.2393739 , 0.73558828, 0.45568838)
rgb=(0.24606968, 0.73890972, 0.45202405)
rgb=(0.25289851, 0.74221104, 0.44828355)
rgb=(0.25985676, 0.74549162, 0.44446673)
rgb=(0.26694127, 0.74875084, 0.44057284)
rgb=(0.27414922, 0.75198807, 0.4366009 )
rgb=(0.28147681, 0.75520266, 0.43255207)
rgb=(0.28892102, 0.75839399, 0.42842626)
rgb=(0.29647899, 0.76156142, 0.42422341)
rgb=(0.30414796, 0.76470433, 0.41994346)
rgb=(0.31192534, 0.76782207, 0.41558638)
rgb=(0.3198086 , 0.77091403, 0.41115215)
rgb=(0.3277958 , 0.77397953, 0.40664011)
rgb=(0.33588539, 0.7770179 , 0.40204917)
rgb=(0.34407411, 0.78002855, 0.39738103)
rgb=(0.35235985, 0.78301086, 0.39263579)
rgb=(0.36074053, 0.78596419, 0.38781353)
rgb=(0.3692142 , 0.78888793, 0.38291438)
rgb=(0.37777892, 0.79178146, 0.3779385 )
rgb=(0.38643282, 0.79464415, 0.37288606)
rgb=(0.39517408, 0.79747541, 0.36775726)
rgb=(0.40400101, 0.80027461, 0.36255223)
rgb=(0.4129135 , 0.80304099, 0.35726893)
rgb=(0.42190813, 0.80577412, 0.35191009)
rgb=(0.43098317, 0.80847343, 0.34647607)
rgb=(0.44013691, 0.81113836, 0.3409673 )
rgb=(0.44936763, 0.81376835, 0.33538426)
rgb=(0.45867362, 0.81636288, 0.32972749)
rgb=(0.46805314, 0.81892143, 0.32399761)
rgb=(0.47750446, 0.82144351, 0.31819529)
rgb=(0.4870258 , 0.82392862, 0.31232133)
rgb=(0.49661536, 0.82637633, 0.30637661)
rgb=(0.5062713 , 0.82878621, 0.30036211)
rgb=(0.51599182, 0.83115784, 0.29427888)
rgb=(0.52577622, 0.83349064, 0.2881265 )
rgb=(0.5356211 , 0.83578452, 0.28190832)
rgb=(0.5455244 , 0.83803918, 0.27562602)
rgb=(0.55548397, 0.84025437, 0.26928147)
rgb=(0.5654976 , 0.8424299 , 0.26287683)
rgb=(0.57556297, 0.84456561, 0.25641457)
rgb=(0.58567772, 0.84666139, 0.24989748)
rgb=(0.59583934, 0.84871722, 0.24332878)
rgb=(0.60604528, 0.8507331 , 0.23671214)
rgb=(0.61629283, 0.85270912, 0.23005179)
rgb=(0.62657923, 0.85464543, 0.22335258)
rgb=(0.63690157, 0.85654226, 0.21662012)
rgb=(0.64725685, 0.85839991, 0.20986086)
rgb=(0.65764197, 0.86021878, 0.20308229)
rgb=(0.66805369, 0.86199932, 0.19629307)
rgb=(0.67848868, 0.86374211, 0.18950326)
rgb=(0.68894351, 0.86544779, 0.18272455)
rgb=(0.69941463, 0.86711711, 0.17597055)
rgb=(0.70989842, 0.86875092, 0.16925712)
rgb=(0.72039115, 0.87035015, 0.16260273)
rgb=(0.73088902, 0.87191584, 0.15602894)
rgb=(0.74138803, 0.87344918, 0.14956101)
rgb=(0.75188414, 0.87495143, 0.14322828)
rgb=(0.76237342, 0.87642392, 0.13706449)
rgb=(0.77285183, 0.87786808, 0.13110864)
rgb=(0.78331535, 0.87928545, 0.12540538)
rgb=(0.79375994, 0.88067763, 0.12000532)
rgb=(0.80418159, 0.88204632, 0.11496505)
rgb=(0.81457634, 0.88339329, 0.11034678)
rgb=(0.82494028, 0.88472036, 0.10621724)
rgb=(0.83526959, 0.88602943, 0.1026459 )
rgb=(0.84556056, 0.88732243, 0.09970219)
rgb=(0.8558096 , 0.88860134, 0.09745186)
rgb=(0.86601325, 0.88986815, 0.09595277)
rgb=(0.87616824, 0.89112487, 0.09525046)
rgb=(0.88627146, 0.89237353, 0.09537439)
rgb=(0.89632002, 0.89361614, 0.09633538)
rgb=(0.90631121, 0.89485467, 0.09812496)
rgb=(0.91624212, 0.89609127, 0.1007168 )
rgb=(0.92610579, 0.89732977, 0.10407067)
rgb=(0.93590444, 0.8985704 , 0.10813094)
rgb=(0.94563626, 0.899815 , 0.11283773)
rgb=(0.95529972, 0.90106534, 0.11812832)
rgb=(0.96489353, 0.90232311, 0.12394051)
rgb=(0.97441665, 0.90358991, 0.13021494)
rgb=(0.98386829, 0.90486726, 0.13689671)
rgb=(0.99324789, 0.90615657, 0.1439362 )
}
}
}
\begin{document}
\begin{tikzpicture}
\begin{axis}[domain=0:pi,samples=81,samples y=81,colormap/viridis,view={20}{55},hide axis]
\addplot3 [surf, point meta=y] {exp(-0.5*x*y)*4*cos(deg(5*x)) * sin(deg(12*y)) + 10*cos(deg(0.5*x*y))};
\end{axis}
\end{tikzpicture}
\end{document}

The outcome is the same for both pdftex and luatex, but (on my ancient system), pdftex requires 65 seconds to compile the picture whereas luatex requires 21 seconds (a factor of 3).
More optimizations of that sort are likely to follow. If some parts of the PGF backend engine where ported to luatex
, the example's runtime would be considerably faster - the remaining part of the luatex runtime is due to plain old TeX code which takes its time.
So, my partial answer is "If you make use of pgfplots
, lualatex
can provide much more speed in texlive 2015".
Some details for those who want to benefit from the improvements: the example benefits from lua because compat=1.12
(or newer) enables a special lua backend
; a partial reimplementation of many expensive sub-operations in lua
.
The lua backend
is a partial implementation. If it is unavailable for some input option, the file's .log
file will always indicate that it fell back to the TeX implementation (and why).
Something else occurs to me: there are packages which are plain unavailable for pdftex: I am thinking of the excellent graph drawing library of PGF. This is not quite related to the question (which is more about speed), but still: luatex has left its "message" when it comes to advantages for end users.
fontspec
approach to loading text and math fonts when compiling under LuaLaTeX. – Mico Aug 15 '15 at 7:59microtype
is indeed slower withluatex
than withpdftex
. This is mostly because of certain operations that have to be applied to all glyphs in the font (and Opentype fonts can be huge!), These loops are carried out intex
while it would be much faster usingluatex
techniques. This will be fixed in the next version ofmicrotype
(due October), which should at least resolve themicrotype
bottleneck. – Robert Aug 16 '15 at 3:31