Thanks for the details on this! It’s kind of interesting that you have both memory and disk maxxed out at the same time. (I’d expect one to hit the limit first and force the upgrade.) It’s mildly surprising (to me) if filling the disk didn’t cause any harm or damage. But looking closely, you only filled the disk right at the end - previously perhaps at about 95%.
On the memory front, it’s notable that your disk i/o fell off at the same time you got the extra memory. That’s a clear sign that the machine was doing a lot of paging activity, which can indeed cause dramatic performance problems.
I see DO say that “used memory is calculated by subtracting free memory and memory used for caching from the total memory amount.” This is interesting, because people often say that their memory is 100% used, and there’s two very different things they might mean.