Very slow preview app experience

I'm finding the online preview experience very slow, at the moment. This is an app, not DPS Preflight

Without tapping on the preview button, the project I am working on is constantly 'previewing' and its taking over an hour to fill the little blue bar as pictured below.

Waiting for the results of any changes are taking an age - and this is not practical at all. You press 'preview' and hope that at some point in the next 30 minutes your changes to cards, collections, layouts etc will show on the device as an 'update' but this is simply not happening. I'm on a superfast network at a major publisher so that's not the issue.