Running WPT is much slower than it used to be.

[glennw]

Using the follow subset of WPT:

./mach test-wpt --release --processes=8 tests/wpt/mozilla/tests/css/

Previously (perhaps 2 months ago?) this would complete in ~70 seconds on my machine. With current master, this is now taking ~175 seconds, nearly 2.5x slower than it used to be.

The attached image shows what the CPU usage looks like during execution of the command. Previously, all CPUs would be maxed out at ~100% for the duration of the test run, so it seems related.

[glennw]

Anyone have any ideas what may have caused this regression? @jdm @emilio @metajack ?

[jdm]

Ironically, I see the opposite - using master on my 2015 macbook pro, it takes 70 seconds to run 560 tests in that directory. Using 121662a from May 9, it takes 139 seconds to run 557 tests.