For a way more detailed look at memory architectures and implementation, check out Ulrich Drepper's classic paper "What Every Programmer Should Know About Memory"[1]
Which just goes to show, hitting memory is a Bad Thing(tm) even when you're running on a slow(from today's perspective) processor like a 68000.
Doing 22kHz generation on a Macintosh is very close to the limit
http://www.6502.org/tutorials/6502opcodes.html
(Zero-page memory on the 6502 was the memory accessed via addresses with a high byte of 0x00. Since 6502 had sixteen-bit RAM addressing, this meant each page was 256 bytes large, so the zero-page was almost as good as having 256 single-byte registers.)