--- Operating System, Graphics card ---
Windows 7, x64, gtx 580

--- Blender version with error, and version that worked ---
r56837 has error

--- Short description of error ---
When the modifier visibility is turned off from viewport, playback speed in the movieclip editor is normal (no viewport visible)... when the modifier visibility is turned on for viewport... the playback speed is significantly reduced. I have a feeling it is trying to load in the smoke for every frame even if the viewport isnt visible

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---
Step 1. Open up a blender
Step 2. create a smoke simulation which is complex and bake it out to a file
Step 3. change 3d viewport to movie clip editor
Step 4. open movie in movieclip editor and press play
Step 5. turn smoke visibility off from viewport and notice the speed difference on the framerate.