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
Edit > Comment
You will see all of the default keyboard shortcuts and customize your keyboard shortcuts under menu
Options > Configure Texmaker > Shortcuts
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.