Thank you for this report, it was genuinely valuable. All three points are addressed in today's update:
The tree bug was a canvas coordinate mismatch under browser zoom. Your "playing in firefox btw" was the clue that cracked it, so thanks for including that. The lag was mostly shadow effects that Firefox handles poorly. The game now auto-detects low FPS and switches to a performance mode (there's also a manual FX toggle in Save & Options). And you can now download the game as a free offline HTML file, since you asked.
As for the UI it's a fair hit. It got a full redesign in this update. Curious whether it lands better for you.
Full changelog here:https://itch.io/t/6628355/changelod?before=3#post-16584813
If anything still feels off on your end, I want to know.