This also happens to me (using the same version of Symantec Workflow Manager) using Windows 7 x86.
It appears that when I switch models, the csc.exe process starts and consumes an entire single thread of my CPU for a significant amount of time. This causes the Workflow Manager to be come unresponsive, however if you wait long enough the csc.exe process will end and the new model appears for editing.
Note that if I have multiple workflows open, only the window in which I changed models becomes unresponsive. The rest are fine.
The csc.exe exhibits the same behaviour when I try to publish the workflows with numerous invocation targets. The time to publish is almost excatly the same amount of time it takes to switch models, and again the csc.exe process starts and consumes a single CPU thread for almost the entire time (the utilisation varies a bit). Sometimes this makes for a very lengthy publish process on the client, however looking on the server where the workflow is being published, the workflow has already been published with the changes well before I receive a "success" notification in the Workflow Manager.
The workaround is to remove all invocation targets from the workflow - switching between models is then instant - but it would be good to have a fix!