You could tell if you screwed something up performance-wise, because the tape would have to reposition and reread if your program was not ready to read blocks coming of the device. With IBM 6250 BPI drives this meant it would overshoot massively, rewind a bunch of tape, and then have another run at it. [0] It was painful to watch and not super good for the mag tape either.

The old movies that show tape drives sort of stuttering back and forth reflect slow program performance. That's not really how they looked for most of the things I worked on.

[0] https://www.ibm.com/ibm/history/exhibits/storage/storage_342...