Right now the jit-test runner's remote target (for testing real android devices) is single threaded. We should try to support parallel jobs to speed up those tasks.
Once Bug 1486004 lands, it will be straightforward to test this on unrooted phones.
At the same time, we should investigate if there should be a slight architecture change to avoid calling adb shell for each test and doing a handshake with the device. Instead it would be nice to upload some sort of runner shell script that reduces traffic over the adb link.