Running 'mach gtest' locally against an x86 emulator, there is a long pause just before the test app is launched and tests run. Most of that time goes to pushing files to the device:

https://searchfox.org/mozilla-central/rev/85ae3b911d5fcabd38ef315725df32e25edef83b/testing/gtest/remotegtests.py#94-104

Currently, with a fresh x86 emulator, the libxul push takes about 30 seconds and the support files take another 60 seconds.

After bug 1486004, we anticipate an increase: libxul push still takes 30 seconds, but the support files take 90 seconds.

(A quick test on a Galaxy S10 took more than 4 minutes to push all files, after bug 1486004.)

Can we do better?

We could select the files to be pushed, place them in a temporary source directory and push the entire directory to the device in one call rather than pushing the individual files. That might be a significant win.