Hello Eckart
Welcome to the forum
A lot of things is happening when you move the frame, perhaps too much. This demands some resources from the processor.
When you drag or resize the following things happens:
* It repositions the small squares(the resize handles).
* It resizes some 50% transparent divs.
* It also adjust form values in Internet Explorer.
I need to take a second look at this script and see if there's something I can do with this problem.