The reason Fast RAM is faster is that it isn't shared between the custom chips. On a 7MHz system it isn't very noticable speedwise since the memory on an A1000/500/2000 is 70 nanosecond which can access at 14 MHz so the custom chips and the main processor just take turns.
The main time you have to consider Fast RAM to be useful is if you don't have enough to fit everything into Chip RAM or if you have an accelerator card. Getting the maximum performance out of an accelerator card involves forcing as much into Fast memory as possible.
