Compared to the other pauses kernels make? Biscuit (POSIX in Go [0]) introduced at worst a pause of 115 microseconds and a slowdown due to GC and safety checks of 15%. But the forementioned systems don't have a kernel/userspace split; Microsoft measured the split made a server 25 to 33% _slower_ [1].

[0] https://pdos.csail.mit.edu/papers/biscuit:login19.pdf

[1] https://dl.acm.org/doi/10.1145/1178597.1178599