Before, I had measured the Doom benchmark on my notebook somehow incorrectly. Now I ran it again, and the score is ~310. I am not aware of any changes in dosbox settings, maybe the notebook switched to the better (of the two) graphics cards automatically. But now the performance difference between the notebook and tablet is very substantial, which could explain everything.
Edit: That low tablet score I obtained because I had limited the CPU cycles to 50000. Now, when I set CPU cycles to Auto, I got the much better score. That was my fault, sorry.