I have a similar problem.
I am running Debian 10 “Buster”, and I have tried both with Flatpak, Appimage and through .deb installers, same result. I can install and run the package just fine, but when I open it, I get an almost entirely unresponsive window.
However, this only happens when I open it in an X11-based window manager, as is my daily work environment (AwesomeWM). I tried logging in to Gnome on Wayland, and Obsidian ran smooth as a hot knife in butter.
To check whether it was an Awesome problem rather than an X problem, I logged in to Gnome on X, and tried to run Obsidian - which was unusable. So I am pretty sure that at least for me, it’s an X vs. Wayland issue.
I am wondering if there is some GPU rendering issue or something that does this?