As the A21 is a budget phone its quite normal for them not to be "lightning fast"
which one of the A21s do you have? the June 2020 model or the A21s?
as its a 170 pound device whilst the spec sheet on the A21 2020 model says lightning fast its still operating Android 10 on outdated hardware and low internal storage combined with 3gb of ram that are almost always running at full use even while looking at your home screen.
I can try and help optimise the settings with you if you like but I just want to make sure your expectations are within the budget range and not similar to the A51/71/90(budget to midrange) or any galaxy phone after the 5th in comparison
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.
I would say probably fastest devices in Samsung A series are Samsung Galaxy A51 5g or the Samsung A71. I use A51 5g and have had no issues with speed whatsoever. Completely surpassed my expectations 😊😊