This is frequently caused by SharePoint Group providers. The slowness is a result of someone registering a Group Provider for a site, but then that site gets deleted or is no longer functioning. So when the K2 Worklist web part loads, it attempts to access this group provider's site, which will eventually time out, resulting in slow response times.

To find out what group providers are registered, open Workspace --> Management Console --> Your Server --> User Managers --> SP --> Group Providers. You can get and test the URLs listed here. 

To remove a provider not in use, you need remove the record from the db. These are located in the K2Hostserver.GroupProvider table. Make a backup of the dbs before altering the data and remove the problematic row(s). Also note that directly modifying data in the db will void your support, so if this is a critical environment (QA or Production), open an official support ticket for this.