Well, within 10-15 minutes you have probably already played most of what the game has to offer at the moment (except for the ending screen), so no need to worry about the rest of the game (for now). I still believe that a performance optimization could help, even for your old CPU, as the game is pretty much completely unoptimized now and there's a lot of potential there. And I have yet to decide on whether to add a save system or not. Ungrow is not one of those endless idle games. It might become longer with updates and then a save system would make more sense, but that is all still some time in the future :)
The game is pretty similar to Peter Talisman and partially inspired by that one, you're totally right about that. But I don't think that's a bad thing, I really liked that game :) And I think they're different enough already and I'll try to make this one even more special with the planned updates.