I remember System 7 being full of race conditions: MacOS was originally designed to get events from the keyboard and mouse and disc and when they started coming from the internet for web browsers it broke down.
System 8 dealt with the problem by putting in lots of locks which slowed everything down and it made it unresponsive, at least if you didn't have the latest machine.
Classic MacOS was doomed at this point because it was designed for 1984, not for 1988 and Apple had to hustle to build an all-new modern OS.
