First off, I searched for this and one came up, but it was for a snapshot and it was resolved for being invalid. For me, however, I am running vanilla. I don't even have any texture packs on, but I don't see how that would cap it to 30 fps anyway.
I loaded Minecraft 1.18.1 and my FPS was capped to 30. It can't be my computer running slow because everything else works fine, and it never goes above 30, not even 31. Just a solid 28, 29, 30. Max FPS is set to unlimited.
Also, when I walk around ingame, it doesn't even feel like 30 fps, it feels more like 15 or maybe even 10.
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