The whole application freezes during upload

Expected behaviour

The application does not freeze, and we can still use it during uploading files to the server, all the job is performed in a background thread without blocking the main thread. Also, responsiveness is not affected.

Actual behaviour

The application freezes during file upload, and almost does not respond as all of the touches and other events are queued in the main loop and are not processed at an expected time, what harms the user experience.

Steps to reproduce

Just upload a file either manually or by a synchronisation mechanism: make a photo or video using your iPhone and then open the app: it will start synchronisation, and this problem will occur.

iOS version

iOS 12.2

App version

Latest on the 19.06.2019, 2.23.7.9, 2019.
The problem described was happening for a half year at least,
so not only this version has this defect.

Server configuration

All versions of next cloud 14, 15 and 16 (not 16.0.1) in docker (so from dockerhub)

Operating system:
iOS, Linux.

Web server:
Nginx

Database:
MariaDB

PHP version:
The one in the official image of nextcloud in dockerhub.

Nextcloud version: (see Nextcloud admin page)
16.0.1

Start by reproducing a manual upload or synchronization-triggered upload on iOS, then trace which upload work blocks the main thread. Done means the upload completes without freezing the application or delaying touch and other events.