For those who may be struggling with latency in Reaper (or perhaps any DAW) as I was, I noticed that the huge latency I experienced went away whenever the track was armed for recording. That gave me the clue I needed... the buffers and prioritizes things depending on what it thinks you're trying to do (obviously something you're playing in real-time should get priority).
To make this effect permanent, bring up the contextual menu for the track that Spark is on. Go to "Track Performance Options" and tick "Prevent Anticipative FX". This is essentially a buffer that does no good for real-time instruments! Read about it here: http://www.soundonsound.com/sos/jul11/articles/reaper-tech-0711.htm
I hope this helps any of you who have been having difficulties with this software.