Navigation button taking long to activate

I have a quiz in which there are  four slides that have videos. When I lock the navigation i.e users follow the order of slides, it takes about 40 seconds for the "Next" button to activate after the video has ended. This is only happening on the four slides with the videos but not the the rest of the slides where users answer questions and  then click "submit" I have also noticed when I change the navigation to free the problem does not occur i.e. the "next button activates instantly after watching the video. 

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