"Durham is only really slow if you're near the bottom of the queue"
Yes and no. For some subjects in some places the queue can be pretty long, which is why I suggested OP check the applicant to place ratio. If there are, say, 13 applicants to a place and your DC is held waiting for a decision till Easter, they have probably done well.
"Plenty of DC get offers before Christmas or just after it, across all subjects. "
But again this depends on subjects and where they are applying. And often the ones kept waiting are those who narrowly missed Oxbridge., or those who are applying for the oversubscribed courses popular with DC who narrowly miss Oxbridge.
I accept that most dont have this problem, but OP was not suggesting this. When it happens, it is really tough on the applicant. All their friends are booking accommodation and getting stuck into revision, whilst mine were still waiting on three replies each.
It worked out fine for us, and I hope the same is true for OP's DS. I got my sort-of revenge moment. Warwick waited till March to give DS a weird Tesco substitution offer. DS is now deciding on which funded PhD offer to take up. There was no interest at all in applying to Warwick. Far too low down the rankings.