The speed limit is "walking pace", but the main reason that such streets are pleasant to live in is that they simply don't work as through roads so the only vehicles you'll see here are those which belong to people who live in the street. You're not going to make any journey quicker by driving through here instead of on the main road. Note that they're also not through routes by bicycle. A woonerf is for living in. It's not a main route for anyone.
Often, people complain that Dutch solutions to creating a cycling and people friendly street-scape might take too much room. This is a myth. In these examples, much has been achieved in very narrow streets and without re-arranging existing buildings. The important thing is to distinguish between where people live and where suitable places are for through journeys to be made.
A correspondent in the comments wondered about how nice it might be if cars that people owned were parked at the edge of the development and therefore not in the streets where people live. This is difficult to achieve with housing which already exists, but there is an example of a newer (1980s) development in Assen where this was done:
Grotere kaart weergeven
The problem with this is that people like to be able to see their car from their home. Newer developments have not copied this style.