Both
A processor first and foremost.
Most decent accelerator cards like the Blizzard have 32bit fast memory slots and the difference in speed is very noticiable with the 32bit memory .
Example: My (now sold) A4000D had a 68030 at 50Mhz but the standard ram installed on the motherboard. My A1200 had a 68030 at 50Mhz too but the Blizzard 1230 had 32bit ram installed and it was noticibly faster then the A4000.
This might be a dumb question but I am relatively uneducated about computer hardware in general, and the amiga in particular (mine was never expanded).
Which will affect an amiga's speed and performance more? Adding fast ram or adding a faster processor ?