Comment 67 for bug 1690719

If you want to fix this problem correctly, then in addition to decoupling the JS engine from the compositor thread, you'll also want to call mlockall(2) in the compositor process (to prevent page faults and swapping under system memory pressure) and run with SCHED_FIFO (or other real-time scheduling class) to make sure that CPU-intensive workloads don't starve the compositor process. You need to make the entire process SCHED_FIFO or you'll get priority inversions with process-wide locks (like the heap lock), so you're best bet is giving the compositor its own process and putting the shell elsewhere.

Everything else is a half-measure. If you want the GUI to be responsive under load, you need to insulate the GUI _from_ load, and the previous paragraph describes how to do that.