This is to do in view mode
If the problem is the save then it could mean you have some custom Event Listeners doing a bad job. Do you know if you have installed some extensions or developed some custom Event Listener code?
Regarding perf, you need to read this page which contains everything to know on performances:
BTW 12K pages is not really large. When you reach several millions it’ll start to be large!
Just make sure you adjust the doc cache as explained in that doc, depending on how may docs are frequently accessed. Thus you need enough memory to be able to contain all the cached docs. That will be the main way to size the memory you need.
But that’s not going to solve your problem.
I don’t have any more ideas except for Event Listeners.