It's a device running Android on a bottom-of-the-range SoC with, according to the description, 5 out of "9" gigabytes of "RAM" running from swap space on the internal storage.
Perhaps Doogee could've ported Android better, but I don't think Android will ever run smoothly on this device.
Android contains a lot of tricks to cache as much as possible in RAM so things like sleep/wakeup and app launching can be very fast. You can see the device take a while to launch a terminal on Debian, that's exactly the kind of thing Android uses all of its RAM for to prevent.
It's a particular choice of terminal, that. I can't help wondering how quick it would be with urxvt or similar. I'll probably pick one up in a few weeks to find out myself.
On the Android side there are faster launchers and faster terminal apps as well. Loading fully-featured, touch-friendly apps just takes a while on hardware like that.
I've run postmarketOS on an old Galaxy Tab II back in the day and that had similar (though much worse) issues. You can open minimal applications quite quickly, but the on-screen keyboard doesn't open and trying to things like scaling are likely broken.
Perhaps Doogee could've ported Android better, but I don't think Android will ever run smoothly on this device.
Android contains a lot of tricks to cache as much as possible in RAM so things like sleep/wakeup and app launching can be very fast. You can see the device take a while to launch a terminal on Debian, that's exactly the kind of thing Android uses all of its RAM for to prevent.