Hey thanks for the report! Are you playing the downloadable version, or the browser version?
Viewing post in Mr. Magpie's Harmless Card Game comments
Ah, yeah that could be it. It might also be something to do with a corrupted save file. You might want to try the downloadable version and/or try clearing your cache in your web browser for itch.io.
I know we've added a lot of unperformant things recently, we will hopefully find the time to optimize them, but even then, dramatic lighting in a web browser is a pretty hard thing to pull off.
Really appreciate the bug report though! Thanks!