The Surround Mixer does in fact use up significantly more DSP than the Stereo Mixer. For example, a Surround 5.1 Mixer with 20 tracks means 20x6, or 120 gain/pan points. A Stereo Mixer could have up to 60 tracks with the same number of gain/pan calculations (60x2). Therefore, make sure that Stereo buses or Stereo Audio tracks are indeed using the Stereo Mixer (and not a sub-path of a Surround Mixer, which will still use the full DSP of the Surround Mixer). To put it another way, if you only need Stereo panning on some tracks, route them to a Stereo Bus and then to the Surround Mixer via an Aux track. This way you are only using the DSP necessary for the Surround Mixer on a single Aux instead of many Mono or Stereo Audio tracks.