Of course for downloaded videos I use VLC but what shall I do with Facebook videos, which require flash player? Those videos are laggy 
And these videos do not run without flash player.
Keep in mind my note about how not all videos are encoded in a way that lets them be hardware accelerated, and not all video players are written to take advantage of hardware acceleration.
That said, *if* hardware acceleration should be used, but isn't -- it's usually because you've got a really old GPU, and either the hardware or drivers were buggy enough that we blacklisted a particular driver version, and/or the card itself.
In general, Facebook videos tend to be HTML5, but it depends, and it's constantly changing, so it's hard for me to make any informed statements there. You can figure out if it's Flash by right-clicking on the video. If it doesn't say About Flash Player at the bottom of the context menu, we're not playing the video.
First things first, make sure you're running the latest browser version and graphics drivers for your system. Also, the output of the DirectX diagnostic tool would give me a good inventory of your hardware and drivers. Both things are covered in the video troubleshooting guide, below. Make sure that you *do* have hardware acceleration enabled, although it should be on by default.
https://helpx.adobe.com/flash-player/kb/video-playback-issues.html
If that doesn't help, use Chrome, and go to chrome://gpu. Chrome has really good GPU diagnostics. It's also the case that if the browser blacklists a card or driver, it's not going to be available to us, either. If you can copy and paste the output of the chrome://gpu page, that would really help me understand what's going on.
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