Insane lag spike for a few moments while starting game, cpu usage maxes out all of my 8 threads and ram usage constantly above 2GB, this makes my PC temporarily unusable until MC loads. This is incredibly annoying as I normally am doing something else while waiting for MC to load and my PC gets too laggy to do anything. The lag is mostly tolerable for vanilla, however this slows down load times and is especially apparent on modded clients with forge installed. I have no idea why this keeps happening as it didn't do this previously.

How to replicate:
This started happening to me when I started experimenting with commands, more specifically the /data modify command. I have no idea why this would affect performance while loading the game but apparently it does.

UPDATE 2022/07/03: I haven't touched this report since I made it in 2020 and I feel like I should have. As many people have pointed out in the comments this is indeed caused by the DFU. It still affects 1.19.1-pre2 although now that I am on a different system (Core i5-9300H with 8GB DDR4 and an SSD) this issue is less apparent, however on lower end systems this issue will probably be a lot more visible.