Keeping 30k emails in your inbox is a risky strategy.
There have been many reports on the forum of forum users inbox contents "disappearing" and despite reporting it to BT it has not always been successful in recovering them.
If you want to keep them in one folder I would suggest that you move them onto a new folder that you set up in your My Folders section and set up a routine where you move your inbox contents to that folder on a regular basis as part of your back up regime, assuming you have one.
This is so bad.
Inbox of my main BT account worked fine for a day, and now it is back to the same 1min lag everytime I click on inbox, or refresh inbox. I can't even more to another foler, say an archive one, because then going back to inbox is another 1minute wait.
It's a pile of brown stuff.
Still badly broken and not fit for purpose.
I am going to have to log another fault/support call.