Commodore went with zorro slots not because of "ISA being slow", they went with Zorro slots because of the pin count on the 1000's Zorro-II expansion port. If you need 100 pins and an ISA slot has 98 (36+62), what do you do? You use a different sized slot to keep costs down on expansion products.
(Everything below this is just blather, the above statement is the only real important one
Amiga 2000's are basically just modified Amiga 1000 (as is the A500) motherboards with the expansion port turned into multiple zorro ports.
The original A2000 was designed in Germany. It was based on an integration of the A1000 motherboard design and the example Zorro II backplane from "Schematics and Expansion Specifications", the A1000 hardware manual. - Dave Haynie. All Amiga's are basically the same computer, sure the later AGA chipset machines were technologically different, but the 500/1000/1500/2000/2500 are all the same beast with motherboard modifications (and even 3000 which is basically a modified 2500 with an integrated flickerfixer). You could even buy a zorro expansion kit for your A500 which gave you multiple Zorro expansion slots under your 500's case. This allowed you to use
most of the same Zorro cards on your 500 as you could use in your 2000 (excluding the accelerator slot and the video slot). These expansions worked with no fancy software, no drivers, no new chips, etc. - just plain old circuit boards with solder trails going from expansion pins outs to zorro slot pins.
That was/is part of the beauty of Amigas, everything worked on different models because they were basically the same machine (you didn't have to write software for ten different video card chipsets, or five different sound cards). And what they did at the time was leaps and bounds ahead of any other manufacturer. :-)
