Well, yeah, a scene doesn't have any spine animation but importing spine causes fps drop (jitter).
Here are pictures of comparison between profiler with spine plugin (left) and without (right).
I also updated my first message with a statistic image, it's the same scene, fps is by 15 smaller than it should be, but it drops and jumps too much, so it looks much worse.
Our project includes plugins like Behavior Designer, ProCamera2D, and a lot of custom made generators (level generator).