When I have 10 or more light bulbs in my views (esp. the 3d view makes a difference), WED becomes extremely slow.
Is that normal, is there something I can do?

Thanks in advance!

What are your system specifications?
Go to view/glx propertys, and turn of the level lightning in the 3D and 2D view.

I've got a dual core 2.0Ghz, 2GB RAM, 512MB Geforce 7900GTX, not the fastest computer on earth, but I think it should be capable of displaying some wireframe bulbs. (?)
Level lighting is deactivated.

Doesn't anyone have the same speed problems?

Maybe you've already tried it, but you could try out some video out options under view options(the GLX.....dll things)

Did you have the latest drivers?