I installed lineageos 21 (android 14) rom on my a32 and it is 10x smoother than one ui so yeah one ui is heavy on lower end phones such as a32 a22 a12 etc
I don’t suggest you buy the A15 - there’s not much difference. Except for the performance and display which differs by little, it’s all pretty much the same. You should consider a higher A series device something like from the newer A3X series or last year’s A34. Or if you can’t afford it then try something new or wait for next year’s A16
The Tecno Pova 5 has a better refresh rate but LCD screen whitch is very bad compared to the samsung a15 90hz super AMOLED screen, On the Tecno U get better camera just by a little bit and a bigger battery and faster charging but I would reccomed the samsung a15 becuase the user experience will be better and u get more software updates.