Should be fixed now. The issue was something similar to what happened in this thread with helper columns: Grist accepting data very slowly - #4 by anais-grist A column conversion had some error.
There is now an easy way to fix this should you encounter it. Go to Document Settings. There is a little skull emoji. It’s a secret button! Click it to force restart the doc… (We should really make it a normal button now!)
I went ahead and did it in your document. It should be working fine now.