SERVER-28210 we tightened the default threshold for Microbenchmarks master branch from 10% to 5%. This seemed to work well for a while. A small number of tests needed higher thresholds, and we've set them on a test by test basis. However, recently we've had more failures which seem to be just false positives, and on a wide range of quite basic tests, such as a dozen update tests.
We will relax the threshold back to 7.5%.