Recently we've received a few customer feedbacks reporting WF4 workflow designer would have bad performance or hang in some cases even when the workflow is relatively small. We've worked with customers and found out that it was actually a WPF issue.
Good news is, the WPF team has already release a hotfix for it. So if you are experiencing bad performance/hang with WF4 workflow designer, I would recommend you to install this hotfix and try again. Although I cannot guarantee it will solve the issue in your case (since the root cause may be different), there is still a good chance that it will help.
You can find the hotfix using either of the links below.
I would also encourage you to download the following .NET cumulative update patch which includes a bunch of other improvements from WPF and WF4 workflow designer.
Have a try and tell us whether it works for you~