Poor SSRS Rendering performance in SharePoint 2010
This was related to two things:

The web servers of the SharePoint farm had no external web access, so the root certificate revocation was failing (adding a delay of 15-20 seconds to paging and activity) - This was resolved by giving them access to the external web via a proxy.

The issue with extra overheads when using SSRS integrated into Sharepoint is pretty well documented and still seems to be a problem - I guess that's part of why Microsoft are disabling the Add-In in the future.