I am involved in development of Sync, so want to clarify few things:
1. We never tested it on Kindle, so hadn't had any chance to optimize for the device. It is nice that it works
2. Sync uses AES256 to encrypt the traffic, we saw some reports that this could significantly slow down the synchronization.
3. It is in pre-Alpha, so there should be issues that we hadn't found yet.
If we might get a help from you - debug log files - to analyze the issue, then I am sure it will work perfectly on Kindle.
Anyway we will touch Kindle at some point of time, but log files could significantly speed the things up.
Thank you testing Sync,
kos