What makes a ROM slow?

[Vaudou]

So I've tried a lot of the ROM's here (CDMA), and it seems that some are faster than others. What I want to know is what makes a ROM fast or slow? Comparing NRG's GTX Sense ROM and XYC's ANA ROM, both have very similar layouts, yet (to me at least) ANA ROM seems to be faster (note I'm talking about each right after flashing and all the necessities have been tended too; i.e. provisioning, xda_uc, quickgps, etc.) than EnergyGTX. I know each have different apps/utilities installed, but both look the exact same in layout just with a bit modified taskbars/slider/icons, etc. Are these factors what make a ROM fast or slow or are there more in depth factors that determine a ROM's speed (like the programming, etc.)? I want to know this because I want to be able to have a simple, yet sexy looking ROM, that is fast. Yet every ROM I've tried seems to be laggy on the home screen, or takes a bit to load a lot of apps, or is slow at transitioning, stuff like that. Is there anyway to change certain things in a ROM to make it faster, or is the "slowness" from the phone/OS itself? Sorry if I'm a bit ambiguous, ask and I'll clear any details up. Thanks for help in advance!