I removed some apps and I think it runs smoother but if i want better battery performance do i need a custom rom? Can i always go back to the stock rom if i ever wanted to?
Sure, just do a nandroid at any time before you flash a ROM and you can revert 100% to the way it was (minus sd card changes). You have to wipe everything for a custom ROM, so you're looking at setting up your apps and what-not.
How lond does a process of installing a custom rom takes? If i want a pretty much stock rom do I just put the file on the sd card and install from there?
Flashing (installing) a ROM usually takes up to ten minutes to do. The easiest thing is to download the ROM with your pc and push (copy) it to your sd card. You always flash a ROM from recovery on your phone. Always to a nandroid backup before you flash a ROM.