Firstly, what do you consider to be slow? That is a serious question, as different people have different expectations.
Next, can you see when it is slow? The sync dialog shows the steps as the sync is done.
For me, while I don't think the sync is very fast, the only step that I think is a problem is the annotations syncing. That can take several times longer than the rest of the sync together. If it wasn't for that, I think it would be finished in less than five minutes.
The above is on my Clara HD. That has 621 purchased books and 7069 sideloaded. It is also where I probably do the most annotations (mostly marking errors in sideloaded books to fix). My Libra H20 with 647 purchased books and 251 sideloaded, completed a sync in about 2 minutes.
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