I don’t know if this helps. I used to have a very long delay before receiving email messages. Later I learned how to solve thaht confiuration problem. Perhaps it could be the same cause for you, so I’ll share my experience.
My email service is with my own personal email provider. Because I didn’t want to use large providers like Google mail, Apple iCloud or MS Hotmail, I pay a small yearly fee (was $5, now $20) to have my own email server and my own domain name.
But email took a long time to arrive, anywhere from 15 minutes to half a day. In particular, sometimes to log on to certain online servvices, they send you an email saying, “Click on the link in the email I just sent you, to verify that you got the email and it’s really you trying to log in. This link expires in [whatever time, 5-15 minutes].” And then the email would show up an hour later.
After trying to get tech support for YEARS (and being resigned to the fact that my email provider must just be slow), someone at the email provider finally realizes that someone had configured my email account for GRAYLISTING.
Also known as GREYLISTING. Not blacklisting, where you just reject spam; not whitelisting, where only known senders’ emails are allowed through, but graylisting, to combat spam. It works like this: My email server automatically rejects the first email that I get, with some error. They figure that, if the email is really important, then the sender will re-send it. The second time the email arrives, my server accepts it. But this can take a long time, and is dependent on how long the sending server waits before trying again.
It’s like deciding that you’re not going to answer any phone calls the first time someone calls; if it’s an important call, they will call back. Yes, you’ll avoid a lot of telemarketing scam calls. But you’ll lose a lot of friends.
The most important thing was that THEY DIDN’T TELL ME that my email account had been configured for graylisting. I had never even HEARD OF graylisting before that one tech support staff told me, and I’ll bet you all the other tech support staff before this hadn’t either. Once graylisting was turned off, my email magically worked, and there were a dozen online services that I could use again because verification email showed up in minutes rather than hours.
I don’t know if this is your problem. If your other email arrives on time, it could be that your email provider only applies anti-spam measures to message classified as more likely to be spam (e.g. if they don’t recognize Delta Chat headers or encryption as such). I hope this helps you and others figure out what’s happening with your Delta Chat messages.