Nextcloud Talk Notifications Delay (15-20-25s)

Good day to you, everyone!
I have such char: Centos 8, Nginx, MariaDB. All the versions of software are up to date including Nextcloud.
I haven’t found such issue or the simular one across the whole forum or in internet. They are all seems like close to mine, but the problem is different.

What i have as information:
Nextcloud Talk on iOS devices (iPhone Xr both if it helps somehow).
If phoneA calls to phoneB then phoneA immediatelly receive push-call message to accept or decline it, quality of call is perfect.
If phoneA in the same chat with phoneB (both displays are turned on), then messages receive immediatelly (up to 1-2 seconds).

What is the problem:
If phoneA is in app and phoneB isn’t and phoneA creates message and send it, then phoneB will receive notification about it only after 15-20-25 seconds (lots of tests, delay in time is between 15-25 seconds).
Also that is interesting, if phoneA sends message1, after that message2 and then message3 (one after another immediatelly), phoneB wait 1,2,3 seconds after message1 was sent, then phoneB open app, all the messages from phone A will be received, but push notifications will be received like: message1 after 15-25 seconds, then another 15-25 seconds pass and message2 ntification received and another 15-25 seconds and message3 notification received.
Sorry for my English. Does anyone know what it should be or what it may be?
I’ve tried already thoughts about http2 adding into nginx config, reloading it but it gave no results.
Thank you very much!