Every time a new unexplored chunk was loaded, the FPS drops to almost impossible to play. I nearly died to a Ghast because the FPS didn't cooperate well. It felt like playing Minecraft at 1 FPS every time the game is loaded up. There weren't many mobs in the chunk yet it still lags. Lag is one of the things that everyone hates.
Thank you for your report!
However, this issue is Invalid.
Simply stating that "My game is laggy" is not helpful for Mojang to diagnose your issue. If you can pinpoint a specific cause when lag happens for you, please open a new report for that specific issue. If you think this might be a hardware issue please contact Community Support as they might be able to help with your issue.
Keep in mind that Mojang is constantly optimizing the game and simply playing the game provides feedback to Mojang on which parts cause performance issues. To learn more about this, head here (section "Telemetry") for more information.
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