I put it down to low latency between the mouse counters and the hardware sprite, and immediacy of visual feedback due to the on-screen gadgets being rendered in a high-priority task distinct from the application's task.
A 40-year-old 7MHz computer can genuinely feel more responsive than a brand new machine thousands of times faster, because of those two things - even if the machine then takes minutes or hours to complete a job the new machine can perform almost instantly.
BeOS was probably the last system which offered that same degree of responsiveness.