Long time delay when sending a large file

There is quite a time delay between clicking on "Send" and the outgoing message appearing in the chat; we should measure what exactly takes so long there.

FTR, there are two more small UX problems when sending a large file, which we should also look into afterwards:

- [ ] The message shortly gets a letter icon right after it is sent
- [ ] It's unclear whether we want to send a read receipt after the pre-message is seen by the receiver, or only after the attachment is also seen