Excellent chat option with very elegant construction.
Text messages are instantaneous. Small images (~200KB) are a few seconds.
It is not really built for large files, but that can take up to a few minutes.
Images and video are downsampled to a smaller size before sending, to save bandwidth. (Quality holds up well and there is an option to attach as a 'file' if full resolution is needed.)
Hold mic-button to send voice messages.
Jitsi integration for video calls.
I am also trying the fork: DeltaLab UI that includes a few extra changes.
The setup is easy.
I would recommend using it on a dedicated email account, only for DeltaChat usage. See the docs.
Setting 'delete messages from server' to 'after 1 week' will make sure your online storage is never going to get full. (Server messages can be deleted once transmitted to all clients installs for the recipient)
One of the best FDroid finds over the last year, highly recommended!