Cause of the issue:

Every time an activity is run, the ActivityLog table in the database is updated with the run status of the activity. Over time, this can cause the database to grow to quite a large size and make the loading of the Manager interface or refreshing of the Manager interface after an activity has been run, very slow.

Resolution:

Upgrade to v3.3.17. 

In v3.3.17 a daily purge of the ActivityLog table is now performed after the first activity for the day has completed.
The purge with remove all records from the ActivityLog table older than 30 days.

If the database has grown to a very large size, then the first purge may take a minute or so to complete.  Subsequent purges should only take a second or two, but will depend on how many records are being deleted and the system resources/specification.

To keep the database at its optimum size, you may also want to perform a regular (maybe once a week) 'squeeze' of the database.  This will compress the database to its smallest size.
The database squeeze is preformed by selecting the FILE > DATABASE > SQUEEZE CURRENT DATABASE menu item.