This is not the best design, you can still go full speed through it. Much better ones actually force you to slow down, ie sub 30kmh otherwise your suspension, tires etc will get hammered a bit (or more). There are numerous ways how to do it, ie central island is wide, island's walls are high and steep, you can have speed bumps before it etc. Its has been done elsewhere and it works.
So it worked, weeded out dangerous driver before it can kill bunch of kids crossing the road on pedestrian crossing. Generally it really works, I mean 100% of the cases where I live. Everybody slows down, normal cars, sports cars, motobikes etc. If you don't, you suffer for your own ignorance.
Indeed, with even a halfway decent sports car, a roundabout isn't a traffic calming measure, it's a little bit of excitement. If you want to slow traffic down, put in an Indian style speed bump. Though in the US you'd probably have vandals taking it a jackhammer to it the next day.
Someone probably died here for this roundabout to be added though.