Symptoms
Workflow Designer is slow to load on the /FrameworkInterop.asmx web service call.
Diagnoses
In the past this was related to the attempt to load unneeded workflow guides, however in this case it seemed like a Process rights issue.
Resolution
The following bug item was logged and the corresponding fix can be applied, which resolved the issue in this case:
Bug 496791: K2Blackpearl Feature Request: K2Blackpearl Workflow Designer : Feature request to enhance the loading of the Designer for the first time - Process rights for current user new workflow