I had this issue myself, managed to fix it by Completely closing minecraft going to http://account.mojang.com/me/settings and removing the link from my account and Twitch, Re-Adding the link between my Twitch Account and Mojang waiting 5 minutes just to be sure it had fully linked then launched Minecraft and was able to stream from then on.
/MrAdder89 - Twitch Global Moderator
similar behavior on MacBook Pro 16-inch 2019 with 64GB RAM - starting Minecraft spikes kernel_task up to 900% CPU and game lag / graphics performance is terrible despite having an 8 core Intel Core i9 with AMDRadeon Pro 5500M with 8GB VRAM, would love to get that fixed, let me know if I can assist. Also adjusting video/Java settings has had no effect.