Removing relations for completed activities to reduce XER

• Is it acceptable to remove predecessors relations for activities with actual start?

No, activities with actual start might still be unfinished and might have some predecessor links still active, also it might mask out of sequence occurrences, both issues might cause errors in your schedule.

If you are transferring data via a database file format it shall be efficient, if using Excel to transfer data it might be inefficient.

I do not believe links will take much memory space, I would never recommend deleting any links, not even for any activity with actual finish, criteria for all activities with actual start don't even think of it.

If you make a test, say duplicate your job as to perform the test on a duplicate and make the deletions, of course on predecessors for activities with actual finish how much the database size will be reduced? How much the XER file size will be reduced? 50% or a mere 5%?

To my understanding the big problem with XER files is that they transfer some data without your knowledge, it is usually required to clean up the data using some utility like XER Manager http://executiveportlets.com/downloads/EP%20XER%20Manager.pdf