For me the performance increase of Nuendo 6 with ASIO guard vs. Nuendo 5 means that I can now run 3 ms buffers for all my sessions all the time, so I don’t need to switch to 12 ms anymore. This is a huge plus. I run hecacore mac pros with Nuendo sessions that include quite a bit of plugs. To be quite honest I don’t care about the performance of the GUI too much.