Performace drop in FlatBoxAA renderperf test for metal pipeline

There is a performance drop in FlatBoxAA renderperf test with metal pipeline from sprint#72[26-July] to sprint#73[9-August].

Tested on mac mini 10.14.6 with default non retina display and with uiScale=2 option.
Result of FlatBoxAA renderperf with the build of 9 August with metal pipeline : 18.24 FPS
Result of FlatBoxAA renderperf with the build of 26 July with metal pipeline : 36.87 FPS

So the performance decreased from 36.87 FPS to 18.24 FPS.