Hello !
Finally, I have found the solution. I updated to last version of Nuendo 7.0.40 and the Midi tracks are seenable now. I made CPU test between Nuendo 6.5.40 and Nuendo 7.0.40 and it’s clear that the last one is more CPU friendly on Mac OSX 10.9.5 ! (About 10%) and the CPU peaks are much more reduced. The Midi tracks and very large projects (756 MB) are better handled too.
The unique bad think is the VCA tracks, FX Tracks and Group Tracks are not shared thru Network in collaborated Project. What’s shame ! I hope I can handle this for my very big artists installation at the Tate in London…
Djengo.