I wonder if fmt.Printf() in particular is slow due to the CGo transition? Assuming there even is a CGo transition when compiled to WASM...

There's no CGO involved when compiling to Wasm. The sometimes slow performance is due to the hoops the compiled code has to jump through to support the Go runtime and goroutine preemption on a single thread.

I understand there's no CGo specifically, but I'm wondering if the Go runtime when running under WASM still has to manage switching out the goroutine stacks for "WASM stacks" when it's calling out through the WASM VM.

Edit: from this comment [1] it sounds like this is the case.

[1] https://news.ycombinator.com/item?id=37501552

Tinygo runtime don’t have g or m threads that’s why it’s Cgo is zero cost