Yes.  That's why there's a periodic gettimeofday() check in perhaps_sweep_siblings() to try to avoid running for too long.  See the commit message for 1563feb3e64c85a5498e992090e99fb4d1825555 for an exploration of the added latency.  The commit excerpts from a script that runs IIRC 4 times.  I show histograms from the 1st run and 4th run in two difference cases, 90% of keys mutated and 9% of keys mutated.

At the time, I was also measuring off-CPU latencies with DTrace.  It turned out to be a fruitless investigation, but I did include the histograms from off-cpu stats in the commit message, but I don't believe they show anything useful.