Doesn't that prevent you from using some of the most useful crates?

Kinda but there is good usage of proc macros and bad usage.

In order to collect reliable statistics on compile times, crates.io would have to be doing the compilation themselves. There's no way to trust telemetry from random users, both due to situational variability (individual machine load, overprovisioned CI runner VMs, overcommitted RAM, etc.) and malicious activity. That would mean a significant increase in hosting costs along with a slew of additional work to filter out spam, malware, and DOS attempts, plus building robust data collectors and aggregators.

As to proc macros, I don't think it's possible to implement derive without them. I think it would be fairly easy to detect their presence in a crate at least though.

Both things are also made more complicated by feature flags.

Could we use crater runs?

we leave poor performance stats and correction as an excercise to the user