Repository Registry seems to validate/check each repository, which can prevent HG Workbench from opening until this check is done.
In my case there was a network repository, which did not exist anymore. So it took the full network-access timeout until Workbench finally opened. (around 10-20 seconds...) which was really annoying. Removing the non-existant repository fixed the problem and workbench opened fast again.
A: Maybe run those checks in another Thread, and do not block workbench from opening?
B: Add an option to disable the repository registry creation/checking/usage at all? (Personally I do not use it anyway)