DeltaChat is not made for for the multiple 100 people groups, but you can make mailing lists, at the moment only unencrypted mailing lists are possible to my knowledge, but schleuder recently added some auto crypt support (Key auto import from email (!426) · Merge requests · schleuder / schleuder · GitLab), so will probably be possible in the future. mailing lists also have the advantage of making anonymous groups possible by hiding the email addresses.
That being said delta chat stores all messages locally, IMAP is more used like a transport protocol, delta chat itself is offline-first, if you have huge busy groups you either:
- need storage space for all the messages on your device,
- or disable automatic downloading of large messages such as images
- or enable disappearing messages in the chat for everyone or autodeletion from your device after X time
Though normally messages are small, like you can get tons of text messages before it becomes an issue, but depends on your available storage space.
Normal groups can be public, you can share the invitation qr code or link, only limitation is that your device needs to be online for the other person to join, but there are bots that can do this for you. (current favourite GitHub - deltachat-bot/invitebot: Delta Chat bot that allows to generate invitation QRs and links for your private groups; older bot with web interface: Public Groups - a bot to let others join your channel)
a. yes
b. yes
c. moderation tools
- cleaning chat: no once people received the messages you can not remove them from their device
- kicking people is possible with removing members, but there are ways to get work around that and come back to the group, so you only really can get real moderation with mailing lists (those are proxies and you decide who can send or join them.)
- authoritative groups with admins are really hard in decentralised settings also see New feature: Group Types
- performance:
- for delta chat the biggest group I’m currently in is 76 members. This group still works nicely, limits depend on header size limits and recipient / rate limits of the involved email providers.
- in group chats the messages are encrypted symmetrically and then the symmetric key is encrypted for every person ans sent in an email header.
In the future this might change, I myself are not really knowledgeable in OpenPGP, but there are interesting developments and innovation happening from what I heard.