How does the ZIP chip work?

["Eric" <englere.geo@yahoo.com>]

I've been wondoring how the ZIP chip can speed up an Apple IIe. I
understand it has a 4 Mhz 65C02 and it plugs into the original 6502
socket.

But, the original 6502 only has half of each 1 Mhz interval (I'm
rounding to 1 Mhz to make it easier to discuss) to access memory
because the video refresh circuit uses the other half. So, the orig
6502 has a 500ns cycle to access memory.

If the ZIP chip does not have it's own memory (does it?), then it can
only speed up that part of the 1 Mhz cycle that is not used for video
refresh. Assuming main memory could run 4X faster (which it can't),
then it could bring that 500ns down to maybe 125ns. This means the
overall processing (including the obligatory video cycles) would go
from 1000ns to about 625ns, which is less that a 50% speedup.

But main memory can't run any faster, so the zip chip must have some
kind of cache? What am I missing here?