Moffat shouldn't really be the problem. After all, even if he personally has become slower at writing, aren't there a whole bunch of other writers out there that can write stories while he script edits them?
So long as Moffat is solely responsible for the major arc episodes of the series (season openers, season closers, mid-season arc keystones), production will always be limited to Moffat's writing speed.
Even though Moffat commissions more scripts each year than he can use in case a freelancer's script falls through, you can't slot a standalone script into the "Wedding of River Song" slot and have it work as a season finale.
The days when JNT and Eric Saward would give someone like Peter Grimwade or Robert Holmes a laundry list of narrative elements for a script are long gone.