Properly built speed bumps are most comfortable to traverse at posted speed, but are very well felt when you're speeding. In my experience they build them well in Northern Europe, less so in Eastern.
Not in any parts of northern Europe where I drive. You are usually (exceptions exist) supposed to reduce speed very much below posted speed limit before passing the bump. For example, posted speed is 50kmh but speed bump is designed to be crossed at no more than 20kmh. If you drive a less comfort oriented car, make that 5kmh instead of 20kmh.
I drive mainly in Norway but hadn't noticed any difference in Sweden or Denmark. If you drive the posted speed here you can count that your wheels won't be knocked off. If you have to reduce speed (say around schools or near poor visibility crossing) then there is a sign for different speed, as simple as that.
You won't lose your wheels going at the posted limit, if it is a 30kmh road. For comfort you probably do want to reduce down to 15-20 in an average car.
If you encounter a warning sign for speedbumps on a 50kmh or 70kmh road you better start braking if you dont't fancy getting airborne.
There are some weird deadlocks in Swedish road regulations that make it unreasonable difficult to change the speed limit in some situations. They sometimes work around that by actively making it impossible to drive the posted limit.
OK, maybe that's a thing in Sweden after all, certainly not here. Just drove a whole bunch of speed bumps on the way to work today anywhere from 30km/h to 50. You'll never encounter a speed bump in a 70 km/h zone here.
Allow me to cordially invite you to test out a speed bump set up at exactly 51.201580°N, 16.972742°E. It's not visible in Street View yet, which adds to the surprise.
