I bought an iPhone 11 less than 2 month ago. Why is my maximum capacity 81% already?

I bought an iPhone 11 on Amazon for just over £200. It came looking untouched everything looking great. It worked great for the first month but the second it started lagging badly and then I went to check my battery health because I saw the memes of how bad it is once it gets less than 80%. I was shocked to see my maximum capacity is suddenly 81% from the 100% it was when i first got it. Not even my iPhone 8 is this bad!! My iPhone 8 that I’ve used for about 2 years and was second hand is still in its 90%‘s so wtf is happening? I feel scammed. The lag is making it hard for me to be able to call anyone, type big amounts of text that I need for school and not only that my iPhone camera is stuck on this overly edited type of style (very soft looking, sharpened, edits my face, doesn’t make anything look realistic at all) I cannot take a picture of my cat even without it looking absolutely awful!!