It seems that SharePoint Designer performs poorly when dealing with anything other than really basic workflows. From what I have noticed, it seems that the time to open the workflow gets exponentially longer with the number of activities present in a workflow. The same can be said with editing those activities, in fact I often spent minutes staring at my screen after updating simple activities waiting for the Workflow to surface back from its coma.
Increasing Performance Tip
The real problem here is the number of activities inflates the Workflow XOML size (which can also prevent validation if too big) which consequently affects performance. In order to validate your Workflow XOML, a web service call is made to the "ValidateWorkflowMarkupAndCreateSupportObjects" method. In my case, the XOML was too big and the validation never completed thus resulted in a server error:
Some re-factoring in the workflow had to be done to ultimately reduce the number of activities as much as possible. Eventually, my Workflow XOML got lighter and validation could be completed properly. I also noticed some minimal performance increase as the Workflow would open quicker and would hang less.
Though I am not saying SharePoint Designer is not a good tool to build workflow, this certainly reveals another limitation developers should be aware of as re-factoring will not always be possible.
