An extremely high number of videos and picture sequences in a timeline may use up a lot of performance for recreating the objects and may slow down editing. In such cases you can hide the thumbnails in the objects.
In the main menu item Timelines click Display video thumbnails and the thumbnails that were visible are no longer displayed in the video and picture sequence objects. I.e. this function toggles between the two conditions.
A fast change-over can be achieved by hitting Ctrl + U.
See also