Playback laggy while all RAM not being used

The playback was laggy for a pretty medium sized score for me so I checked my task manager and my computer was using exactly 50% RAM constantly. So then I tried opening Reason 10 and there was no problems at all and the computer started using about 63% RAM. I know this is not a hardware issue as I have 16GB of RAM while MuseScore only uses 1.3GB when I have this perticular score open. Is there a setting in MuseScore that limits RAM usage and how do I change it or is this some kind of bug with dual channel RAM?