This article is from the Macintosh system software FAQ, by Elliotte Harold firstname.lastname@example.org with numerous contributions by others.
One of the Memory Control Panel (or General Control Panel in
System 6) settings is the mysterious cache, Disk Cache in System 7,
RAM cache in System 6. This is memory the system sets aside to hold
frequently accessed data from the disk. The cache acts like a 7-11
for your hard disk. It's quicker to get a quart of milk at the 7-11,
but it costs more so you don't do all your shopping there. And the
7-11 doesn't have everything you want so sometimes you need to go
to the A&P (your hard disk) instead.
Unfortunately the caches in pre-7.5 system software really aren't
all that fast. In these systems the RAM cache would more appropriately
be called the RAM thief. Its effect on performance seems to be much
like the canals of Mars. You have to want to see it before you can.
The caching algorithm has allegedly been improved in System 7.5 but
I haven't seen any hard evidence of that yet.
However there are a few applications and extensions such as
Dayna DOSMounter that actually make use of the cache and will run
much faster when it's turned on than when it's off. Thus I recommend
setting your cache to 64K, turning it on, and forgetting about it. I
hope that in 1995 most Macintoshes have enough RAM that they don't
need to worry about losing 64K.
If, however, your Mac is a IIsi running a color monitor from
the internal video, then you may possibly speed up your Mac with an
appropriate cache setting. The IIsi and the IIci use system RAM to
store the video image on your screen. (Other Macs with internal
video have video RAM separate from the main system RAM so this trick
doesn't apply to them.) The internal video competes with the System
for use of this RAM; and that competition slows down your Mac, just
like two children fighting in the back seat of your car adds an hour
to the time it takes to get to the beach. To stop the fighting a
smart parent will put one child in the front seat and one in the back.
A smart Mac owner will put the internal video in the front seat and
the system in the back seat. To push the system out of the front seat
set a IIsi's cache to between 384K and 768K which will take up all
the space in the front seat not occupied by the internal video and
force the system to sit in the back. The exact value depends on the
type of monitor you have installed. Experiment to see what works
for you. Unfortunately this trick doesn't work when virtual memory
is turned on, but if you're using virtual memory you're probably more
concerned about saving memory than gaining speed anyway. There's
also a bug in the System 6 cache code that may cause a peformance
hit on disk access if the cache is larger than 128K so this trick is
more likely to help Macs running System 7, but again experiment to
see what works for you.