When comparing total test task durations per test platform, Android 4.3 stands out at the top, or near the top of any list. These are the "Android 4.3 API16+ opt" and "Android 4.3 API16+ debug" tests seen on treeherder, running tests against Firefox for Android.
These tests run on an Android armv7 emulator, on aws. Code run in the emulator is slow. Most tasks run with a tc worker type of gecko-t-linux-xlarge: an m3.xlarge or c3.xlarge aws instance type, more expensive than many desktop test tasks.
Some test suites are particularly slow. On debug, the jsreftest suite runs in 100 chunks and still most chunks require over 60 minutes to run: over 100 hours duration per jsreftest run.
Significant effort has been put into trying to find a more efficient version of the arm emulator, but without success. Newer versions of the Android emulator and newer versions of Android seem to run even slower than what is in use.
An alternative is to run on the Android x86 emulator, with kvm acceleration. We do this for the "Android 7.0 x86 opt" tests, which run on packet.net. Currently jsreftest runs there in a little over 2 hours. But that test platform was put together for geckoview testing and those tests run geckoview test apps. So the existing tests are not a replacement for Android 4.3: Android 4.3 is testing Firefox for Android; Android 7.0 x86 is testing geckoview.
Could we test Firefox for Android on the Android 7.0 x86 platform? Probably, but there would be considerable effort required to "green" tests on the new platform and that's probably not justified if Firefox for Android is near end of life. Our current strategy is to wait for Firefox for Android to move to ESR.