Unless visiting a stream with only very few (up to about a hand-full) messages, the mobile UI never loads the topic/message list. It remains in the stylised "message list loading" type of screen (fancy spinner). To me, this seems possibly related to #4033, but it may be different (more severe?).

In our environment pretty much all users experience this on the mobile Zulip client on both Android as well as iOS. The web and desktop clients all work fully as expected, with a very responsive UI.

Side note: After an issue with the RabbitMQ queue handling on the server (issue zulip/zulip#14862) at the time we have resolved to upgrading the server to the current Git master (beginning of May).