This widely depends on the program you are doing, your goals and the person lifting. At some point, the lifter MUST learn how to struggle though. And if one is a beginner or not cognizant enough to know bar speed or stay focused, bar speed becomes difficult to measure as the 2nd rep may be slower than the 6th rep.
So really - for the vast majority of people reading this, the answer is NO, don't terminate based on bar speed during a PR set. However, the majority of the training sets should be done with EXCELLENT bar speed (BBB, BBS, 5x5/3/1, FSL, etc.). It's a huge component and I still can't figure out why people totally and absolutely ignore this aspect of training. its like why people don't run or jump, I guess.