Phones are... tricky. This is the first game we've made. I was expecting it to be more like a low end pc just with different resolutions... it was not.
I'd say version 20 is your best bet. in version 19 we reduced the texture size so that it would run better. version 20 got a lot of quality of life improvements people had been asking for for a while as well as a massive amount of bugfixes. But it'll be a while before that makes it to the free version.