Good day Tobias,
thanks for a precise description and tests. I investigate how this is done in new MapsForge library and I have to say, very nice idea and system how it's made. Unfortunately it required quite a lot of work in current library (or let's say Version 3 of old MapsForge). I'm currently thinking if it's better to invest time to improving old modified library or do and commit some optimization changes to new version. Both has it's +/-.
For now, I've at least improved DPI scaling, so now values of density used for scaling will be rounded on all devices to 1, 2, 4 values, so if you will use sizes for your images as 8, 16, 32, 64, you will be fine. Unfortunately my device I use for daily use has density that cause no such troubles, but quick test in emulator and it seems to work fine. Ok for now?
Alternative should be to use new system of mapsForge V4 maps + V4 themes, but seems that difference in speed is still quite huge.