Change line 404 in 3rdparty/guzzlehttp/guzzle/src/Handler/CurlFactory.php
increase 1000 to 10000
In lib/private/App/AppStore/Fetcher/Fetcher.php
On line 98 change the timeout from 10 to 30 or 90
in lib/private/Http/Client.php
On line 66 change the timeout from 30 to 90
These changes migh work for you if your connection is slow. They will also invalidate the code checks so nextcloud will complain on the update page.
They will also disapear when you upgrade nextcloud to a new version.