Are you using one of the standard templates? or a custom template?
Can you verify that the 3.01.14 script ran succesfully? specifically the creation of the indexes on the repository categories table?
you can try executing the sproc on the Host->SQL page to see if the problem is in the database or the page. First, determine the module id of your repository module, then try executing this in the SQL page
| exec grmGetRepositoryObjects modid, '', 'CreatedDate', 1, -1, '', -1 |
where modid should be replaced by your module id, and see how long it takes to come back. That will retrieve all repository items for all categories.
If you want to test just a single category, replace the blue -1 in the query with a category id and it will return all the items for that category.
If the query runs relatively quickly, then the problem will most likely be in your page or template so you'll have to look elsewhere. If the query runs longs and appears to be the problem, then you can use some SQL troubleshooting techniques to view the execution plan and make sure your indexes are built properly and being utilized.