Same here.
I've just installed RoundCube (on top of hMailServer), created a single test mailbox and I find the interface being slow in general (2 to 3 seconds delay to compose a new message, or 1 second delay to navigate through messages). I have not found what is causing RoundCube to be slow.
My box is a brand new, freshly installed Windows Server 2012 R2 with a Xeon CPU running at a mere 3% on average. RAM is 6% used with GBs of memory available. Emails and database (SQL Server) on a SSD RAID. Tried locally on the server, ruling out network latency/speed problems. Everything else (including other PHP web sites) hosted on that server are lightning fast, with instant response times.
Perfromance Monitor showing very little CPU or Disk I/O (a few Kb/s by SQL Server, php-cgi at 2%) when navigating through RoundCube's interface.
What could be the issue? Is PHP or RoundCube doing some kind of throttling?