Why MP4 Files Play Slow on a Computer

By Aaron Parson

Several factors can cause an MP4 to play slowly, lag or skip.
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MP4 files contain video and audio data for computer playback. One of the most common video file types, "MP4" actually refers only to the file container – the data inside may be encoded with any one of several formats, such as H.264. Therefore, even while one MP4 may play fine on your computer, another may run slowly or stutter. In many cases, slowdown during video playback is caused by outdated video drivers or codecs, running a video too high-resolution for your computer's speed, or poorly optimized video files and video players.

Drivers and Codecs

Your computer utilizes several pieces of software to play back MP4 videos. The first is your graphics driver. This file tells your video hardware – either a discrete video card or the integrated graphics chip in your processor – how to handle every type of video program you run. An old version of the driver may have errors that lead to slow playback, so always keep the driver updated. Most video player software also relies on codecs. These files decode the compressed information inside the video file to play it back, and missing or outdated codecs can cause playback errors.

Alternate Programs

There are many types of video files, and not every video playback program handles each type perfectly. Different players use different systems to decode video – some don't even rely on codecs – so trying another player may solve slowdown. On Windows, Windows Media Player natively opens MP4 files, but if they play slowly, try downloading another player, such as VLC Media Player, Media Player Classic or Apple's QuickTime Player or iTunes.

Damaged Files

If only one particular MP4 plays slowly, it's likely that the file itself has errors. Video file errors can manifest in several ways, such as slowdown, skipping, stuttering or the inability to use the seek bar. If you downloaded the file from the Internet, try redownloading it in case the file was corrupted during download. If you were given the video file directly from the creator, ask them to test whether it works for them, and to render it again it if not.

Slow Computer Speed

If no other solutions alleviate slow playback, your computer may simply be incapable of playing the video in real-time. Video playback – especially HD video playback – uses a significant amount of processor power, so older computers may suffer from unavoidable lag. Check that no other processor-intensive tasks are running simultaneously, as these will degrade playback. Such tasks include computer games, virus scans and photo or video editing software.