unicode-math + siunitx + lualatex = long compile times. Why? [closed]

Question

The following MWE, compiled under several situations (with appropriate lines removed) gives the following timings (with all output files removed between runs), as timed by Measure-Command on Windows 7 Powershell with TL2014:

Engine   | unicode-math?   | sisetup options             | Time
-------------------------------------------------------------------
lualatex | unicode-math    | per-mode=symbol-or-fraction | ~53 sec
lualatex | unicode-math    | per-mode=default            | ~5 sec
lualatex | no unicode-math | per-mode=symbol-or-fraction | ~2 sec
lualatex | fontspec        | per-mode=symbol-or-fraction | ~4 sec
xelatex  | unicode-math    | per-mode=symbol-or-fraction | ~4 sec
xelatex  | no unicode-math | per-mode=symbol-or-fraction | ~2 sec
pdflatex | no unicode-math | per-mode=default            | ~1 sec
pdflatex | no unicode-math | per-mode=symbol-or-fraction | ~1 sec
--------------------------------------------------------------------

Why does adding the per-mode=symbol-or-fraction with unicode-math increase the compile time by an order of magnitude in lualatex?

Binary versions:

LuaTeX, Version beta-0.79.1 (rev 4971)
XeTeX 3.14159265-2.6-0.99991
pdfTeX 3.14159265-2.6-1.40.15

---

\documentclass{article}
\usepackage{mathtools}
\usepackage{unicode-math}
\usepackage{siunitx}
\sisetup{
    per-mode = symbol-or-fraction,
}
\begin{document}
\begin{align*}
\text{at $p=\SI{1.0}{\MPa}$:}\begin{cases}
v(T) &= v_1 + \left(T - T_1\right)\frac{v_2 - v_1}{T_2 - T_1} \\
v(\SI{220}{\degreeCelsius}) &= \SI{0.2060}{\meter\cubed\per\kilogram} + \left(\SI{220}{\degreeCelsius} - \SI{200}{\degreeCelsius}\right) \frac{\SI{0.2275}{\meter\cubed\per\kilogram} - \SI{0.2060}{\meter\cubed\per\kilogram}}{\SI{240}{\degreeCelsius} - \SI{200}{\degreeCelsius}} \\
v(\SI{220}{\degreeCelsius}) &= \SI{0.21675}{\meter\cubed\per\kilogram}
\end{cases}\\
\text{at $p=\SI{1.5}{\MPa}$:}\begin{cases}
v(T) &= v_1 + \left(T - T_1\right)\frac{v_2 - v_1}{T_2 - T_1} \\
v(\SI{220}{\degreeCelsius}) &= \SI{0.1325}{\meter\cubed\per\kilogram} + \left(\SI{220}{\degreeCelsius} - \SI{200}{\degreeCelsius}\right) \frac{\SI{0.1483}{\meter\cubed\per\kilogram} - \SI{0.1325}{\meter\cubed\per\kilogram}}{\SI{240}{\degreeCelsius} - \SI{200}{\degreeCelsius}} \\
v(\SI{220}{\degreeCelsius}) &= \SI{0.1404}{\meter\cubed\per\kilogram}
\end{cases}\\
\text{at $T=\SI{220}{\degreeCelsius}$:}\begin{cases}
v(p) &= v_1 + \left(p - p_1\right)\frac{v_2 - v_1}{p_2 - p_1} \\
v(\SI{1.4}{\MPa}) &= \SI{0.21675}{\meter\cubed\per\kilogram} + \left(\SI{1.4}{\MPa} - \SI{1.0}{\MPa}\right) \frac{\SI{0.1404}{\meter\cubed\per\kilogram} - \SI{0.21675}{\meter\cubed\per\kilogram}}{\SI{1.5}{\MPa} - \SI{1.0}{\MPa}} \\
v(\SI{1.4}{\MPa}) &= \SI{0.1557}{\meter\cubed\per\kilogram}
\end{cases}
\end{align*}
\end{document}