Hi Steen,
To answer your question, yes, if there really is such systematic slowdown of the PB IDE that you mentioned and Chris confirmed, then certainly we would resolve that before general availability (GA). Please keep in mind this is a beta so please don't assume the final release will be like this.
Also, we suggest taking a look at the C# IDE (SnapDevelop). The performance of that should be good, and we will do further tuning of it before GA. In fact, in some environments we find that the beta version of SnapDevelop is even faster than official release of Visual Studio.
Regards,
Armeen