For the sake of closure - every story needs an ending.

My
BTC client has now synced up after two hours, 27 minutes (started two weeks out of sync), very much slower than the DASH client, as expected.
CPU usage had peak bursts around 60% on a core i7, but was generally not bad. Download demand swamped my rural broadband capacity after about the first half hour, knocking several antminers offline. Disk usage seemed fairly steady at almost half a Gig(!) according to Task Manager - I'm not sure whether to believe that figure.
What several of us are reporting (I think) is that the
BTC client has become very weak when it has to catch up. It is well-behaved once it is in sync, but it has become tempting to leave any
BTC wallet "hot" or to stop using it altogether. Ouch!
I agree that this performance hit seemed to start at about the time of the "stress tests" (and the intense blocksize debate) but I have no opinions on actual causality - lots of variables in play. Anyway,
BTC has a serious problem atm and DASH appears not to share it.
It is a bit of a lop-sided argument to claim that bitcoin's client/wallet syncs slower than Dash's.
The bitcoin transactional volume is probably 100 times bigger than Dash. For two weeks that's quite a bit of data to download.
Comparing the two are almost pointless as the amount of transactional volume Dash has is much smaller than Bitcoin collectively.