Please keep in mind: when multiple videos are being mixed together, they follow the rule set by the "When Combining Video" setting in the Video Tab of the Isadora Preferences. If this is set to "Scale to Smallest Image (Faster)" then all of the images will be scaled to the smallest resolution of all the videos being fed to the Multimix actor.
In other words, if you had three videos – 1920 x 1080, 720x576, 320x240, the output would be 320x240\. This means that the 1920x1080 video, for instance, is being scaled down to 320x240 -- this can be very heavy on the CPU.
You will get much better performance if all the videos are the same resolution.
When you send the video to the Stage using a Projector actor, any scaling is done on the video card. So this is very fast.
Hopefully that makes things clearer.