I wonder if it’s not an AsyncUpdater issue, but a text rendering performance issue that adds up when rendering larger amounts of text.
I’ve been seeing 3-4x slowdowns of text rendering performance on JUCE 8.
Is the updater smooth and consistent when you render just a few characters? 1 message per ms is probably 16-32 messages per frame (depending on your fps), so each frame in your example is rendering quite a bit of text.