When I was digging down to a supposed lush cave, I dug into a mineshaft which caused my game to almost crash because of the severe lag I had encountered. I couldn't do anything for a solid minute. I am on a 3 year old computer and it's currently handling 1.18 very well, but that lag spike gave me a scare.  

Moving across new chunks also gives me a bunch of painful lag because of the caves and mineshafts etc generating under me while travelling. My FPS drops to 2.