Lag in 1.17.5 worlds when moving them to 1.18 pre-release Versions: 1.18

Lag when generating new terrain in pre 1.18 world on 1.18 pre-release

What I expected to happen was...:
little to no lag when generating new terrain in a 1.17 world

What actually happened was...:
my FPS dropped from over 60 FPS to 16 FPS when loading new chunks

Steps to Reproduce:
1. use a 1.17 world and generate some chunks
2. go to the 1.18 pre-release open the 1.17 world 
3. generate new chunks