Current numbers on 3.7b3:

no tracer: Mean +- std dev: 414 ns +- 10 ns
trivial tracer: Mean +- std dev: 792 ns +- 16 ns
monitor tracer: Mean +- std dev: 1.62 us +- 0.12 us
max switch tracer: Mean +- std dev: 3.06 us +- 0.12 us
hub switch tracer: Mean +- std dev: 2.16 us +- 0.04 us