Deactivate subrole slow down FCPX

Hi,
End of the third week editing on a 52' documentary. Today old MacPro (Mi 2012/32Go) was slow : when i tried to lower the level of a clip during the playing, FCPX stop, when i made an insert i had to wait a few second before to see the clip inserted.
In my project i've edited, but deactivate in the index, the Mic Cam Subrole.
As soon as i activate this subrole, FCPX is again performing like a young me on a new mac pro ! I can do all i want during the play of the project, inserted a clip is immediate. As soon as i deactivate the subrole FCPX (10.3.4) again be slow !
This behavior occur even if i don't visualize the audio rules.
So i've now to let the subrole activated and deactivate the clip inside this subrole. (PS : I edit the mic cam for the sound mixer).