It's a problem on multi-output USB interfaces for a lot of people. I suspect system configuration issues but I don't think you should have to take a barebones system approach to get decent timing. I mainly sequence in hardware so I haven't gone to great lengths to get to the bottom of it.

DYI http://www.gearslutz.com/board/electronic-music-instruments-... although you'll have to wade through some non-techie voodoo interpretations if you peruse the whole thread. Also, bear in mind that this is a particularly fussy demographic. I sometimes think if you are that obsessed then maybe MIDI isn't the protocol for you, but OTOH there isn't much in the way of OSC hardware on the market :-/ It's notable that there's a bit of a CV renaissance underway though, a lot of people prize timing/resolution over versatility.