I don’t actually remember logging into this site in the past, but if I said it was running great then I would be comparing that speed to the other 1,000’s of websites/web hosts/web servers that I have logged into around the world over the years. The fact that your other sites are performing well/normally is a very important factor in the equation. That tells you that the problem is isolated to this 1 particular website and is not a general host server problem.
This site is a Live Production site that you probably cannot afford to disable and turn off things to troubleshoot extensively since that would impact visitors to this site. If it was me this is what I would do. I would install a Development site with all the same things installed on that Development site and then one by one eliminate things that could be the root cause of the problem. In other words, reverse engineer the Development website.