Yeah, we don't track perf regressions like released browsers so often we land something that negates the performance benefit. The current servo releases are just alpha, so it's not too important to stay on top of, but we should probably start caring about this more.

We had a similar issue with Stylo (Servo style system in gecko) recently where there were bugs in the parallelism code making us slower than gecko. Fixed, now we're faster again. We only recently started tracking performance properly, and it was caught and fixed in a few weeks.