benchmarks & add zstd info to docs

[LouisHaftmann]

[AnomalRoil]

As a datapoint, maybe, using zstd, running on GCP with Kubernetes, we're seeing upload speeds of ~20.6 MBs/sec on our runners, not sure if that's "normal" or not, but for reference we used to be at more than 150MBs/sec when using Github cache servers which aren't in the same datacenter at all, so that feels very surprising.