Add 'greeking'.

When you zoom text out too much all that is left are some blobs of pixels.
In such a case instead of using the slow drawing text; just draw a rect or a line. This speeds up the painting immensely since the side effect of zooming out < 20% is that you see a lot more text on the screen at the same time. This makes that responsive again on all systems.
Note that I calculate the amount of pixels that the text line would take, so this is the most correct manner of doing this I can think of.