Sven, If you can lay your hands on a Raspberry Pi (
https://en.wikipedia.org/wiki/Raspberry_Pi) you can see what can be done on an ARM 7 CPU using Raspbian Debian based OS. I have an old Pi 2 (current model is Pi 4) which is about 3 or 4 years old. I have updated it in the last few days. It runs the same version of gretl as Userland on the phone. It imay be of interest to note some of the other programs that it can run. - Octave 4.0.3 (Userland 4.0.3), Mathematica 12.0.1 (free on Raspberry Pi), Maxima 5.38.1 (native android version is 5.41.0), R 3.3.3, Tex Live 2016, TeX Studio 2.11.2, Calibre etc. The CPU on the Pi 2 is slow and I would not like to do a lot of work on it. The Pi 4 is much faster. Maxima on android is very usable. The terminal versions of gretl, R and Octave are usable on a phone. The phone screen is not big enough to use GUI versions.
If I get time over the next few weeks I will see if I can compile a current version of gretl on the Pi.