Thanks

The Scene Preload will load Videos that have their Id loaded into a Movie Player (as outline in the article I linked).
That is the recommended method, but as Woland has mentioned, the depreciated Video Preload actor is still available, but requires even more care be taken to avoid filling your RAM with unused media.

But where is the video preload actor in Isadora ? I just find scene preload actor.

Since you mention that you have added preloading actors, I suggest you read (if you haven't yet) this Knowledge base article: https://support.troikatronix.c...

It will help you know the limitations of the preload actors (there are some situations where they may not work, or can cause you problems if not used carefully)

change "optimize" input on the Movie Player from "interaction" to "performance" doesn't seem to change anything (and I use interaction mode for multi sound output).

I stop crossfade between scenes, the fade of jump actor is set to 0. 

I use also preload scene actor (one preload actor for the next scene).

I also disable full frame antialiasing in preferences.

It seems to be better now. 

I'm thinking about the video preload actor, but I can't find it in tools...

Thanks

It might be worth checking that you have the latest Hap codec installed.

Next to that, the obvious thing is that the files are pretty large, so they require a lot of data bandwidth. 
When you do a transition (if it involves a cross fade) double as much data is being transferred, so often this is where users will see some stuttering in the playback, as the requirements double.
Your machine is new and should have a nice fast drive, so this is less likely an issue, but still worth watching  (do you have other things running at the same time that might be using the same drive?)
It looks like you are not doing any interactive manipulation of the video plaback therefore, I would suggest you change the 'optimize' input to Performance, this should change the Playback engine to AV foundation rather than quicktime (seen at the 'pb engine' output). Give that a try and see if things improve.