It's not prohibitive, but it was enough that I refactored some code to use future instead of thread to avoid it.

What was your thread doing and how many did you create?

The delay happens at require time. Just adding the (:require...) clause to the ns declaration adds about a second of time.

I'm sure there's plenty of stuff in async that makes that delay reasonable (if it's needed). I was just wondering if anyone had looked into what causes the delay.

I think there’s a thread pool created on demand

It’s the macro stuff, has nothing to do with the threads

Ghadi actually has a speculative refactor that loads the go loop stuff on demand, avoiding the delay if you’re not using go’s but I’m not sure if that’s common enough to be worth the trouble