Unfortunately, this is not something we will be able to fix. The undertaking it would require to apply optimizations so this scenario would go much smoother, would simply take to much time. Even then, most computers would probably not be able to handle it that well anyway. There are simply so many titles that are contested in this conflict that it takes the game a very long time to actually calculate everything that happens. We tested this a few times, and a high end computer is able to handle it. It just takes a long time for it to calculate and apply everything. You can always try the leave the game running when attempting this, to see if it actually can get through the process (and again, this can take a very long time). But I cannot guarantee that it will work.
If it doesn't work, and you still want to be able to complete your save game, another alternative is to take a White Peace. When you declare a new war, try and siege down a portion of the Holy Roman Empire. This should work better since there should be fewer titles being contested. Therefore you should be able to do it in three or four wars instead.
I know that this might not be want you want to hear, and for that I apologize. But as I mentioned, fixing this issue would require much time and effort, with no guarantee that it will actually solve the issue on the majority of computers people use.
