Help minecraft lag problem?

Help minecraft lag problem? - 1
Added (1). i just bought a new laptop hp envy x360
spec:
i7 - 5th
8 gb ram
1 TB pc storage
GeForce 930m
So on small screen I can get above 60 - 200 fps easy with maxed ultra graphic but when I switch to windowed or full screen it get me massive lag to 50 - 20 fps. I totally don't know what's going on here. So if you can help me here that would be awesome.
Thank you have a nice day

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