Incredibly slow and laggy Macbook Pro, every click lags
I have no reputation to comment, so I will give my answer:


While it's hard to believe, the logic board thing in the Apple Store may have something to do with your lag. If I was you, I'd definitively go on the Apple Store that made the process and ask for some re-check. However, as you said it's been some time, the problem should not appear now, but when you first turned it on. I'd still go on the Apple Store.
Do you have secondary disks, like  a Bootcamp one for Windows? I'd check them, maybe repair them.
Run MalwareBytes for macOS: https://malwarebytes.com/mac-download/
Erasing your Mac to the originals seems like the best option if the answer is not viruses or disk bugs. I'd go for this one after going to the Apple Store