Use the "switch" button on the left of the clock-shaped icon to enable optimization of your video if you're planning on using it as "master timeline" (see article on how to use a media as "master timeline" ).
Enabling this option allows you to instantly refresh video frames when you move the timeline's playhead while editing, making it even more accurate when coordinating your animations with the frames in your master video.
Notes :
To use this option when loading your media, switch the button on BEFORE launching the media download.
If you did not switch this button on for your download, the media can still be used as master
Enabling (or not enabling) this option does not affect the accuracy of the animations and interactions while viewing the project. It allows for greater comfort when creating and editing, but does not impact the end-user experience