You can try adding Optifine to see if that will help, Minecraft is not multi-threaded natively. The only reason you see the game running on several cores is because of your OS and/or CPU itself intelligently multi-threaded it.
If no mods, check to see if Xbox Game Center is running, or Xbox Game Bar. Both are notorious for the degradation of game performance.
Minecraft won't be as good as it can be because it runs in a Java VM.