What makes a custom ROM faster?

[FormerPalmOS]

There are some known performance tweaks (registry edits for file system caches, disabling some unnecessary services, etc.). These tweaks can be made to a stock ROM as well. So my question is this: What about a custom ROM makes it faster than a stock ROM?

One item I can think of is increasing the amount of free program memory so that the above-referenced cache sizes can also be increased, however increasing cache sizes reaches a point of diminishing returns.

Other things I can think of - using a newer build of WM (assuming the newer build is faster), using newer drivers for certain hardware components (again assuming the newer drivers are faster).