I'm using a Harmony 880 remote with my USB-UIRT, and there is an option for setting the time for inter-key delay in ms. Just wondering if you had a value (in 100ms increments), that is ideal for the USB-UIRT. I think I've got somewhere around 300ms in there right now, but things don't seem to move very fast when pressing the buttons on the remote quickly.
The remote codes that I'm using send alternating codes, both of which, have been learned on each command inside of SageTV. (i.e., for "Play" this requires two codes, each key press, it alternates between two different codes.) This probably isn't important, but I thought I would point it out.