Can confirm I'm having this same problem. My GPU is an RTX 2070 Super. Would get a stable fps at a 24 chunks render distance when now I get an unstable 30fps at all distances with vsync on, fluctuating between 30 and 50 with vsync off. The GPU and CPU also are sitting at less than half usage, like Brook Chisnall reported.
Affects version 1.18.0