[nigelm]

Looking at the iPhone console logs you can see it dropping the connection to WiFi although not a lot of indications why.

As calumma suggested, putting the phone into airplane mode, enabling WiFi, and syncing always works - and very quickly. It is of course re-establishing the WiFi link when this happens. However when airplane mode is switched off (WiFi still on or just started if I had not explicitly started it in airplane mode), then OmniFocus goes back to very sluggish sync, and a tendency to knock the phone off WiFi entirely (leading to no sync).