We spend a huge amount of time on each test run looking for changed tests. On my local machine, if I run: `time mach mochitest browser/components/preferences/in-content/tests/browser_subdialog.js --e10s` I get the following output: mach says that the command took 15 seconds time says that the command took 1m8s This, and visually watching console output, is telling me that the setup time for mach test is taking up a lot of time (potentially >45s!). Gregory told me that this was on a roadmap for Q1 2016 to get fixed, but said that there wasn't a bug on file for it yet. Filing now to make sure this doesn't get forgotten.