Interestingly, I don’t have JULIA_NUM_THREADS on my main user’s env, but my inspecting the julia’s runtime I can see that indeed it’s set to 4. Is that the default somehow? Or maybe it’s in some config file that I’ve edited at some point in the past? I’ve check that I don’t have a ~/.julia/config/startup.jl, so that’s not it. It’s probably somewhere in Atom settings. But I think you’re on to something.
EDIT: Just checked that when I start Julia from the command line, JULIA_NUM_THREADS isn’t set and Threads.nthreads() returns 1. So it’s probably Juno/Atom somehow.
EDIT2: This seems to confirm that Juno has this behavior: How to change the number of threads? - #9 by BeastyBlacksmith
EDIT3: Found it. In Juno it’s in Settings > Julia Options > number of threads. Set it to 1, re-running