Sitecore is not capable of streaming media assets like video. When a user stores a video file in the Media Library, the browser must download the entire file before starting playback. Since even a small video file is several megabytes in size, this can destroy a visitor's page performance immediately. If there are a significant number of users on your site, a single Media Library video on your home page can take down your website entirely by overloading the Content Delivery server's ability to respond to requests.
Use a 3rd party video streaming provider like Brightcove, Vimeo, or even YouTube to host your videos and integrate them into Sitecore-hosted pages using their "embed" style players or JavaScript APIs. These 3rd party players can optimize the video's size base on the size of the player on page, and the user's available bandwidth, ensuring they get the highest-performance experience.