I use texmaker and some time ago I found, by chance, a way to comment many lines at once, but I can't remember it now. Can somebody remind me of that?
To comment several lines at once in TeXMaker, select the lines and click Ctrl+T. To uncomment, click Ctrl+U.
To follow up on Torbjørn's comment, you can see the mostly used keyboard shortcuts under menu
You will see all of the default keyboard shortcuts and customize your keyboard shortcuts under menu
One of the shortcuts I find helpful is Ctrl+Space which toggles between pdf viewer and editor, especially when I use a lot of applications and workspaces at once.