Increasing the feed rate will have as much an impact, possibly larger, as lowering the spindle rotation rate. The longer duration the cutter spends in the vicinity of a particular spot on the work piece, the more heat it deposits there.
A bit late to this forum, but is there anyplace i could look to get a tutorial on exactly how to mill my board? Ive used the black rectangle method suggestion and it said it would take 40 hours and that seems like a long time since ill have to do several runs