For me mindfulness is far more relaxing than studying, watching a TV series, working, programming, etc (all "active" things). Never mind socializing which is the opposite of relaxing to me, ALWAYS (autism-related). Although it differs per technique (some are easier and fit me more than others).

Especially the effect after it makes that work more fruitful, since for various hours I'll be less tense and more focused. For example, it'd make socializing more relaxing.

Those are 2 very valid reasons for me to call it relaxing. Then again, I find running and listening to certain music also relaxing. Because after a while, I get in an automated flow ("trance") akin to meditating.

The 3rd reason is that if I'm rather tired, mindfulness can potentially get me to sleep. I often (almost daily) do it before I go to sleep, and it aids me to get to sleep. I much prefer it over: overthinking, fantasizing, or masturbating/sex.

Its already painful for me to meditate whilst sitting instead of lying but one gets used to that. Its just untrained muscles getting trained. As long as you don't strain yourself, you're fine.