IIRC the fastram is rated at 80ns and the ROM is rated at 280ns so that is 3.5x faster.
Fastram supports filling the instruction cache with BURST-mode but IIRC the ROM chips do not. So that makes fastram at least 4x faster than icky ROM chips.
I used to change back to a 1.3 kickstart because Audition 4 and some other apps were never compatible with the new 2.x kickstarts.