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@lx @marco @threemaapp The multi-device protocols of Signal and Threema function entirely different.

AFAIK Signal took the approach to have senders send N messages for N devices of the recipient.

Threema took the approach to have senders send 1 message for all devices and let the recipient's device reflect this message to all other N-1 devices through a different channel.

Both approaches have their pros and cons, e.g. regarding simplicity, consistency and metadata. And although I generally think Threema's approach provides the better tradeoff, one con of it is that it makes forward security pretty hard to synchronise.

That's not the whole story why it takes so long though. In short and without going into details, the reason is a mix of above mentioned, tech debt and the forward security protocol not being designed with multi-device in mind. Also, we started way later.