Thanks for your answer.
I did a few tests yesterday and realized, that the edited files always have the same compression rate and sampling conversion (44,1 or 48 kHz) as the original file. But only when the source has an CBR. If it has a VBR, then the compression rate is CBR and beyond the original rate. I can't explain it much better.
So that I get it right: WavePad alway re-encodes the edited project, even when it's already an MP3? The process lasts always long, but the new file with the same sampling conversion and compression rate made me think, that the app only takes so long, because it has to implement fade ins and outs....