JS is executed in a single thread, but optimized JIT compilation happens on background threads. So if you don't have free cores in addition to the one the JS i

JS is executed in a single thread, but optimized JIT compilation happens on background threads. So if you don't have free cores in addition to the one the JS is running on you'll end up running in interpreter or baseline JIT for longer before the optimized code is available. Whether that affects scores depends on how the benchmark is scored and how long the lag is before the optimized code becomes available, and so forth.