Well, yesterday night I decided to give it a try and deployed a test Server 2008 R2. Roundcube was definitely and noticeable much slower in IIS... I did everything I could to speed it up but nothing gave me much of what I would expect from it. I have Roundcube in Windows Server 2003 with apache in production and it is serving @200 users on daily basis without any slow down. In fact it is as fast as in my Linux servers... this with wincache on some installs and eaccelerator on others deployments (PHP NTS, Apache2.x.x, suhosin, mod_security and all the other good stuff) yet it is smoking fast.
Any IIS Guru that could provide a hint here?