These files contain test runtimes for various suites across different platforms. Each JSON file corresponds to a single test job in production and has the following format:
{ '<test id>': <average runtime> }
These files are being used to normalize chunk durations so all chunks take roughly the same length of time. They are still experimental and their format and/or file structure are subject to change without notice.
The writeruntimes.py script can be used to generate a runtime file. You must specify the suite for which the runtimes are to be generated, e.g.
writeruntimes.py -s mochitest-media