AuthDirFastGuarantee is too low, harming performance
Right now the bandwidth cutoff to guarantee the Fast flag (see #4484 (moved)) is 20KB. That means anybody with 20KB or more of capacity gets the Fast flag for sure.
But our design had in mind that we'd adapt the assignment of the Fast flag to grow as the relays grew, such that the top 7/8 of the relays got it. The guarantee was intended to be a security defense against a Sybil flooding the network with enough lying relays to push out most honest relays.
At the time we picked 20KB for the guarantee, that was the value for the 7/8 mark. But right now moria1 would vote the Fast flag only for relays with 32KB of capacity or more, if it weren't constrained by this 20KB guarantee.
I think we should change AuthDirFastGuarantee to 100KB while we await performance analysis to answer questions like the ones posed in #3946 (moved).