Performance of Designer as well as of any client/server application may depend on 3 factors: database, network, client. Your database server may be too weeak or not tuned well, network connection may be slow and at last client PC may not suit Designer's requirements. If you don't experience problems with other designer tools, it's probably your PC problems. But in any case analyzing Designers schema (you may use RAU for this) shouldn't do much harm.
Regards, Dima