We were considering skipping Delft and using the remaining part of the day to check out Rotterdam, but the increasingly congested traffic convinced us to stop. We ended up doing a blitz tour of the town in one hour, before getting back on the road so that we could get to Ghent on time. In addition to medieval buildings and two large churches (Oude Kerk from 1246 and Niewe Kerk from 1396), Delft has canal houses with doors opening into the canals (not sure if those are actually ever used) and many tourist shops.
Delft was worth stopping at, but it didn’t save us from Rotterdam traffic, since we had to leave before the end of the rush hour if we were to get to Ghent before 10, as we agreed with our hostess. Stop-and-go traffic is great training for stick-shift driving, though perhaps a bit extreme for the first day.