LGPL will be great for MyBB!

[ZiNgA BuRgA]

I haven't really tested those, but I don't think Javascript is really meant for syntax highlighting.  This stuff takes a bit of effort from the system, and a purely interpreted scripting language (some newer browsers may be doing some JIT compilation or similar) isn't really designed for this stuff.
Also, you should cater for users on older hardware.  At a minimum, I reckon someone with an Intel Atom N270 @1.6GHz processor - it should run reasonably well with a large template.