Why does my Apple IIe run so _fast_ ?

I have just played with an Apple IIe (Model A2S2128X) I was given. It is obviously "souped up" as everything runs so _fast_. Most games are impossible to play on it. I just tried a pinball simulation and the ball just bounces around at 100mph! I ran a BASIC graphics extension demo and the bits which were supposed to show "slow" drawing to the screen were real fast, and the "fast" draw was instantaneous. It is not hard to guess that a chip called the "ZIP CHIP Model 8000" plugged into the lower left of the motherboard has something to do with it! I came into microcomputers through the Commodore line, and have never known much about Apples, and I have not accumulated enough reference material to find something about this add-on. Another thing with this unit is that it happily runs DOS 3.3 but any ProDos disks are greeted with a "UNABLE TO LOAD PRODOS" banner. So it will not run Apple Works etc. There is also a PlusRAM II 1MB RAMCARD which is fully populated with 1MB RAM. I suspect that card must have cost $$$ in its day. What do I need to access this memory?Another slot holds a tiny card called a Berkely Softworks IRQ Manager. The centre slot contains a AIIE 80col/64K Memory Expansion card. The remaining slots are the more common Disk II Interface, Mouse Interface and "AII Parallel Interface" cards. Anything anyone can tell me about what makes this IIe run like it does? Phil in Brisbane, Australia.