Automatic video stop on SDCX class 10 card I' ve been recording clips with a Canon 80D and a Lexxar Profesional x633 SDXC card 64 GB class 10, 95 MB/s max read speed. Video format was MOV at 1080p, 25 fps. 

I never had a problem for almost a year with 80D and two years using this Lexxar SD card. But yesterday I just took my cam, (which was off and cool) tried to film, immediately saw the low recording speed bar adviser, and video stopped automatically. Same happened once and again.

Is my card is losing performance due to intense use? Or might the issue be related to used storage (around 75% of SD total was filled, mostly with cr2 raw photo files and JPEGs, at moment of filming).

I don't know if it makes any difference, but battery charge was at its half. Video stopped just some seconds after start, so file size issues are out of debate.
You could try a new card. Could be the issue. My guess it's the camera and the heatsink on the sensor. 

Try a different card first to see if that solves it. Card's can go bad. But with the Canon line (and all DSLR cameras really) do have issues with overheating- especially as they age. 

If a new card doesnt fix it, you can try to service the camera, perhaps it just needs a good blow out/dusting of the heatsink. 

Canon, Sony, Nikon, most models have had some sort of issue at higher resolution video/frame rates with heat issues, and as the cameras age the cameras don't behave as well as when they were new.