Ben, are you set up on a mac to be able to try this out?
It seems like the browser embedding might be doling out fewer threads on OSX for some reason.

Mircea, who reported this to us?

I ran into this problem on MacOS as well. Using Chrome's profiling tools, it always seems like the compile tasks had 50% CPU utilization. I had surmised that we were spinning up too many threads and because of hyperthreading we were not overcommitting the CPUs, but I never got to look deeper into it.

Adding Gaurav, who reported the issue.

Note that deserialization is also slow in that same context (workers)

Please note that this issue can be easily reproduced using simple wasm in webworker which is attached in bug:
 https://bugs.chromium.org/p/chromium/issues/detail?id=811373

Note the difference between the last timestamp and first timestamp shown on console. This test case doesn't do anything useful and just a wasm loading example. 

This bug happens in chrome canary 66 as well. 

 Issue 813259  has been merged into this issue.

+ bbudge, Bill could you look at reproing this?
The speculation has been that it might be we're giving fewer threads to compile on OSX.

I'll take a look.

Bill, wanted to re-up this on your radar.