1.16.4, 20w48a, 20w49a
Windows 10 Minecraft: Java Edition 1.16.4 AMD Ryzen 7 3700X 8 core 5 out of 16 GB of memory allocated NVIDIA GeForce GTX 1050 Ti Java version 1.8.0_271 (64-bit)
Steps to reproduce:
Create a world
Acquire a piece of Cobblestone to unlock the Stone Age advancement. (Can also allow cheats on world creation and run /give @s minecraft:cobblestone)
Experience a clientside lagspike.
What I expected to happen:
Expected to not have a lagspike once I unlocked the Stone Age advancement.
I tested this on 1.16.3 and 1.16.3 doesn't have this issue, furthermore 20w48a seems to be affected by this issue as well. Might be an issue regarding initializing advancements for the first time since game launch because unlocking an Advancement, getting the lagspike then unlocking a different advancement or creating another world and unlocking the Stone Age Advancement again doesn't cause any lagspike. Have not tested this on a server.
Attached a video just now. Here I create a new world and open up the F3 menu to show my FPS at the top left. Then proceed to give myself a piece of cobblestone. Right as I get it, FPS goes down all the way down from 70 to 66 then 39.