What have changed in pandas 1.5?

I upgrade pandas from 1.34 to 1.5. It is very strange when I run the code for my everymonth investment analysis work. It reduced time when I run the code like "pivot", "stack", "merge". It used to take 10hours, now takes 5 hours. But it increased time when I run the code like "group by rank". It used to take 1hours, now takes 8 hours. What happen to "Pandas group by"?