You know, in all honesty, I notice framerate issues more in modern shows. A lot of modern directors seem to think that if you use the right digital techniques, audiences will be too dumb to notice that you're just dragging stuff across the screen or using frame interpolation instead of having someone draw something.
In older shows you tended to see problems with wildly off-model animation before you'd see a framerate drop during motion. (Of course, you saw plenty of sloooow paaans and close-ups where only lipflap was animated.)
An example I like to use is 70s Getter Robo having more actual frames of animation in its OP sequence than the relatively recent New Getter Robo's OP. Actual episodes of 70s Getter are far less consistently on-model, though.
There are also some defects that are just endemic to pre-digital animation, like characters not being able to interact with backgrounds seamlessly. In some really old shows, you can also see scratches or other defects on the cels when you view them in high resolution.