To me a boring movie is one that is mostly a constant flux of extremely predictable events with near zero content to analyze (e.g. a Marvel superhero movie). Tarkovsky is nothing like that, in fact I think his movies are the exact opposite of that.
I didn't say I was necessarily bored. But most people find art movies boring, and Marvel movies exciting. I can try to get the world to adopt an idiosyncratic definition of "boring", or I can use words in their public meaning. Movies with easy-to-follow plots and lots of excitment are not boring. Dreamy movies with messages that can't be summarized in five messages are boring. Fine. But sometimes boring movies are still good.