Is everything else running well? 2-3 minutes? That is very slow.
Closing a ticket doesn't do anything with attachments, so changing storage backend shouldn't change much change there..
Are you using a remote smtp server? It might be waiting for closing notification messages. But you'd likely see that for any notification.. hmm. Weird.
Have you got anything in the logs? Is the server really busy or straining for resources?
Do you get any messages in the browser console? It might be a scripting problem. Pjax should timeout long before that though.
Maybe you can step us through the troubleshooting you've already done?