Reduce jscore execution variance on the waterfall

We currently observe high variance in execution times for the jscore task on the CI loop. While jscore usually takes about 5 minutes to run, sometimes it takes as long as 15.

We would like to add jscore to the commit queue tasks, and we could do so at 5 minutes, but not at 15.

If we can reduce the variance and get the runtime stable at 5 minutes, we could greatly expand the coverage of the commit queue.