DH slows down dramatically

My setup:
Windows 10 (TS150 E3-1225V6/32GB/1TB SSD)
Docker (24GB, full cpu)
Crafty4 (min8GB-max12GB)
Fabric 1.20.1
C2M
Distant Horizon 2.3.3-b

At first it pregens like crazy ~1300cps, then it drops to 50-30 or so for no reason that I know of. It happens after around 200-300 chunk radius so I suspect something gets filled.

Is this a the expected behavior? Where to begin with troubleshooting?