One of the major bottleneck, if not the major bottleneck when syncing a large folder in Sink is KMime's parsing.
The fetch job delivers KMime::Messages which is for Sink rather useless. The fetch job parses the complete MIME structure, when then dump everything into a file, and then parse it again to extract the properties we need.

Ideally the fetch job would just deliver a bunch of QByteArrays that the client can then parse as needed. As a quick measure it would perhaps be possible to not call parse() on the message so we can keep the interface (we would have to see if that would resolve the problem already).