My documents has been the standard location for game saves for probably 20 years. You can say it shouldn't be and you might even be right, but that's the reality and Microsoft KNOWS that's the reality. If appdata is really supposed to be for things like game logs, configuration and saves, can you explain to me why it's hidden by default? How is the average person supposed to copy saves? Back them up? Edit their configuration settings? This is exactly why my documents became the default.
I personally have un-installed onedrive and had it break my file browser such that the onedrive button was still there and any time I accidentally clicked it the file browser froze. I don't think you can chalk up every instance of onedrive slowing down a pc to user error, maybe some other software doing something unexpected + some user error. But again, many people are having issues caused by a software they never wanted. I should be able to be the worst dev ever and do whatever "incorrect" things I want, regardless of what a software I never installed and can't get rid of thinks.
It's also just unexpected behavior, you're telling me just writing to my documents will lag my pc? What if I'm transferring files to my documents while doing something else, now I get massive lag spikes for the crime of putting files somewhere on my computer...? And what if it's pictures I absolutely do not want on Microsoft servers, like medical photos? Am I just required to know Microsoft put software that uploads it without me ever consenting?