OK - one more thought. If you haven't already tried this, try connecting to the IMAP server from a different (network) location. A good trial would be to use K9 on an android phone over your mobile provider's network (make sure you turn off wifi on the phone first). Or (possibly difficult at the moment in lockdown) try connecting from a laptop over a public wifi. You could of course also set up a wifi hotspot on your phone and connect through that from your laptop.
The point is to remove your local network from the equation. If no-one else is complaining and your ISP is not helping, then the problem /may/ not be their end.
I've used the generic email app that comes with my phone, downloaded all the internet options (I always use WiFi, so hadn't bothered with this till now) and it's as quick as anything. So, something my end is stuffed.
Key point for me there. I just got asked by my ISP to test it on my phone and would have done so over WiFi.
But, as I have two other emails connected to Thunderbird (this one I'm using now and another gmx address) and they are fine, how do I track the problem down? It must be something specific to the isp related addresses. (Folders compacted, cache reduced, emails moved, etc, btw.)
Bev.