Well time of initial sync actually depends on number of devices and number of records.
We are encrypting each record and generating decryption keys for each individual device connected in your synchronization group times records. Then those keys are distributed to devices from a device which created the encrypted key for a record.
This mechanizm is quite slow and with increasing complexity of your synchronization group is getting even slower but it guarantees that only trusted device can read the record and no one else. This process also avoids man in middle threat as key is encrypted with information only generating device has (it is never exposed to public) and decryption uses information only receiving has. So even if the attacker would get a record and then some decryption key (which is protected by cipher mentioned above) he can't use any of these information.
As I mentioned we know it is slow and gets slower for big synchronization groups, but only in initial phase. We are thinking of a way how to visualize the progress, but for most of our users this is not an issue as their typical setup is 2 devices (max). So I understand your question and I agree with you it is not optimal, but that's how it is and we are not going to address it now or not even anytime soon.
