Layer-backed backingstore flush copies whole backingstore on each flush

Painting on a layer-backed window on macOS (which is apparently unavoidable now when building agains the 10.14 SDK?) performs poorly because of QImage::detach copying the image. This is related to QTBUG-63559. The commit message for that issue indicates that it isn't expected to be fixed for layer-backed windows.

Is there any way to avoid the copy or disable the layer-backed backing store? I tried setting QT_MAC_WANTS_LAYER=0 without success.