I have posted a few times about journal performance. Make sure to have a look at those.
This one is specifically about open the journal line form being slow. I tested with a 40,000 line journal. Opening the form took about 7 minutes.
I ran the code profiler against it and found one particular method that took way too long. taxAmountJournal().
It ended up being a display method. This particular display method shows up on the form as “Calculated sales tax amount”.
After right click and hiding the field, the load of the form is instant. Same when jumping (focus) from line to line.