someone already wrote the solution to this question in the forum.
Anyway...you can find now that option in the INTERNAL TAB of Preferences (just press SHIFT key when you click on PREFERENCES and you'll see the Internal Tab).
Write in the blank box "SHOW PROGRESS" and you'll find the option that is set to TRUE by default.
Actually, you don't need to go into Internal preferences. Right click in preview window, at the bottom choose Preferences ... and it's one of the checkboxes near the bottom.
Tom
Exactyl!
The option that we can found in the Preview Preferences is "Display Frames in video preview windows during playback", this option is useful to have both internal and external preview (on an external monitor), but it doesn't change anything during rendering.
In VP9 the option you need to remove preview during rendering has been moved in Internal Tab.