Rendering stalls in glyph cache

I am observing non-smooth rendering with an AMD Z430 GPU. The long frame times appear to occur due to QSGDefaultDistanceFieldGlyphCache::storeGlyphs() (in particular the call to resizeTexture() appears to be slow).

Changing QSGDefaultDistanceFieldGlyphCache::useWorkaroundBrokenFBOReadback() to return true greatly improves the situation. However, there are still significant missed frames.

Adding a QElapsedTimer to storeGlyphs, and flicking a list with text, the following times are initially reported (in ms):
178, 46, 72, 66, 69
with the workaround enabled, the times are (in ms):
21, 22, 30, 22, 21