From a report on the MacWorld Expo in 1989: One of the most important developments

Accelerator cards for the Mac, “believe it or not”. This was presumably one of the first time such cards could be developed. Two prerequisites had been met: first, the debut of the “Open Mac” architecture of the Macintosh II a few years previously (March 1987), and secondly a processor advancement (from the 68020 that shipped in the II, to the 68030 in the IIx and SE/30.) Of course, previous closed Macs such as the Plus could be upgraded with special clip-on chips which boosted clock speeds, but 1989 was the first year that accelerators could be introduced based on the Apple-approved NuBus boards.

Now we’ve come full circle and uprading any recent Mac is pretty much impossible once again

This is a fun site. If nothing else, vist it for the design.