I'm seeing the same issue with the v0.10.23 release. Both machines are running Windows Server 2012 R2.
The shared folder has about 48,000 files (10 GB).
When the web UI is open in Chrome, it transfers VERY slow -- about 1 small file every 10 seconds. When the Web UI it's closed the transfer goes very fast.
What's strange is that I first shared a smaller folder with about 4800 files (600 MB) and that transferred quickly, even with the Web UI open. Then, when I shared the second, larger folder it slowed to a crawl.