After much testing, I think I found the issue. If a testcase has a large number of variables, then the opening or saving of that testcase gets very slow. If I create a testcase with no variables, save is very quick. I just add variables, then it becomes very slow to save. The rest of testcase can be empty.
I am now creating a new testcase and slowly adding variable of different types one by one to see when the saving gets slow. I will let you know the results soon.
Results: after adding the 5th variable (I used String, Number, String, Number, String) it starts to get slow to save. I close the file and reopen and it is slow to load.
Something changed in variable handling to cause this.