Cache as Cache can. Unlike dynamic ram, static ram does not require the pesky and time wasting task of a refresh cycle. This is a state in which the processor sits and waits for the slower memory to circuits to catch up with it. The commands used most often are kept local for quicker access and unused commands are dismissed so the data that is needed can take it's place. Simms come a few different sizes ranging from 256K to 512K to 1meg.
Many machines come stock with none at all.
Machines with this lonely simm slot can purchase one of these, pop it in, and in minutes start enjoying 20-50 percent increases in cpu performance. It is more common for Power Macs to have these slots than it is for Quadras and older models. One of these can be coupled with a Clock Booster to gain even more speed. For those wishing to go realy fast, upgrade cards offer the best levels of acceleration money can buy. While not at all cheap, these little rockets provide power to spare.