Avoid pixellation of footage while editing a mask

Folks, I[m new to the forum, and also english is not my native language, so I hope to make my problem clear.

When I edit a path applied to a footage, while I[m moving for example a point of a path, my footage becames very pixelated. This slows down my workflow, because It makes difficult to fine tune the position of the path

Any clue_

Best regards