What does good urls dropping in core web vitals (GSC) mean?

I am talking about the chart you find if you go GSC -> Core web vitals, and you see chart with your current urls. I have a lot of pages indexed (100k+) according to google. Problem is: the 'good urls' keeps dropping, it was 100k, it is now 10k, (poor urls / urls that need improvement are 0). I am unclear what does this actually mean. Is this something to be worried about or it is normal.