The A21s is marginally more powerful then the A21 with a better camera, but the a21 isn't sold in the UK so he must be talking about the A21s. I don't think Samsung marketed that phone as lightning fast, because the A21s definitely isn't. It may be a decent phone for £160, however, it is not going to be fast and you are not gonna find anything 'fast' at this price point. It may be certainly usuable however chances are it may just not be keeping up with what you are doing. 32gb of emmc 5.1 storage and 3gb of ram wasn't exactly great back in 2019 even, and the processor in the A21s has a single core Mali GPU with no fast CPU cortex A7x cores. As
@Phoenix142 said, there may be some things you can do however a lot of it is likely down to the hardware. As a side note, at £170 if you find a galaxy m21/m31 that would be a better performing device, however I'm not sure if you would.