Issue
When opening the Workflow Preview page, it takes longer than expected for the page to load. Additionally, the error can found in the ULS logs:
Error in resolving user 'USER@Domain' : System.Runtime.InteropServices.COMException (0x8007203A): The server is not operational.
Resolution
Use the stsadm commands for setting the properties to be limited on a particular Domain (where the user lives) and the specific Domain under your Forest on a multi trusted AD environment:
stsadm -o setproperty -url http://<WebAppName> -pn peoplepicker-distributionlistsearchdomains -pv <domainname> ?stsadm –o setproperty –pn peoplepicker-searchadforests –pv domain:<domainname> -url http://<WebAppName>
Additional Information
poor performance on people picker search in SPS2010 in Microsoft TechNet