Greetings. I so apologize for the noob question, and im embarrased to say ive never flashed a custom rom before. 3 years with ppcs' and ive always stayed with stock

When you guys say the rom is very fast or quick, whats that referring to? The overall interface, the web, or generally everything? Im really considering taking the plunge into flashing for the first time, andi was just hoping someone could explain that to me. Thanks in advance guys,
dschoenike