Thanks for your answers.
I tried your app a little bit more. Using wifi makes map loading faster (a little bit). Perhaps I have reached my 200MB limit and now I have only GPRS speed till end of the month.
Sometimes, there is no map. Instead, a rectangle shows "...more OSM coming soon"? Is there no map material for the region?
One suggestion: is it possible, when zooming, to display the bitmap from the old zoom level, resized/resampled, until the new material is downloaded and displayed? This would be nicer than a gray background.
Oh, still one question: do you use cache on the internal memory, and if, how much?