Hi, Pierre and others.
In my experience, just let sync finish its task and you're ready to go. The
fact that it may take some time may be due to large amounts of data
(extensions, history etc.) to conciliate between many computers.
Once you sync every device that you may have, the next syncs will be quite
faster.
And, yes, I have experienced this with architectures like PowerPC where I
didn't use iceweasel for some time and, since they seem to lack JIT
compilation of Javascript, the sync took a lot of time.
I would say that this is expected behavior.
Hope this helps,