Some solid advise on how to find what is going on with high I/O wait.
As a personal addition: I’m maintaining a few servers for some smaller projects, and one of them was severely slowing down after a certain midnight process. Tools like iostat and iotop helped me find the root cause. In my case it was basically that memory was too low and too much was written to swapdisk. But something else might be going on. This article explains pretty well what to look after (and 4 years after, still solid advise)