This is a bit of a "how long is a piece of string" question. It depends on the developer workstation, which file they're currently working on, etc. That said, we're talking about incremental compile times here, not total project from-scratch compile times. A lag of a few seconds really makes a difference to developer comfort - I'd say, when it's an issue, we're talking about rust-analyzer responsiveness (mainly due to cargo check on save, which can be disabled) of a 3-8 seconds, and delays in running unit tests of 5-20 seconds. I work on a super beefy ThreadRipper monster so I'm less affected, but colleagues on under-powered laptops suffer a lot.