It might be a duplicate, but I didn't quite find the answer, so what is more efficient - using just a plain video element (if i dont need support for ie8, for example and all the fancy stuff), or using some plugins for video like video.js?
My main concern is load time - is there something that makes video files load faster when using video plugins?
With a plugin (be it Flash or a framework like video.js) you have the load/initialization time of the plugin itself to consider, though that will be largely minimal compared to the pre-load time of the video. Some Flash players have a fairly large .swf file though most (like most of the .js frameworks you'll come across) are pretty well optimized these days.
The largest chunk of time, with either, will be the pre-buffering to get to a state where the video can play through without further buffering. This will be impacted by the resolution, framerate etc of the video itself and optimizations like the positioning of the MOOV atom (if it's at the start of an .mp4 for instance then the browser doesn't have to read/cache the entire clip before it will start playing)
Flash (a plugin) may have better support for different types of media (for instance if you want to use a fMPEG format like HLS or DASH) but may restrict the devices/platforms you can run on. HTML5 and the
<video> tag will - as you point out - only be available on more modern browsers and with or without a player framework to extend it offers a lot of flexibility (and capabilities are improving with time). This is a good overview of pros and cons.