Investigate and fix performance (on multipath)

[flub]

We've noticed a severe single-path perf regression before. We should fix that. We should also compare our previous perf and new perf and make sure they're on par.

[dignifiedquire]

With #3624 performance for iroh is either better or the same in all local benchmarks

[matheus23]

#3624 is merged. I'll go ahead and close. Can be reopened if I misunderstood sth.