Searching for the code snippet from that commit brings up that this Apparent serious JUCE painting issue in Big Sur - #39 by djb-2 is the relevant thread describing why this change was introduced.
Note the quote there:
It seems now you have new evidence to offer on that and I suggest joining the thread.
As a workaround it appears that maybe attaching an OpenGL context (i.e using the OpenGL backend) might help performance (though the colors would be a bit off if not using the SR branch)