i do 400/500 fps but always i get a drop for one second on my fps 5/3 and minecraft freezing for one second and then restart on 500 fps why??

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What I expected to happen was...:
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What actually happened was...:
I do 400/500 fps but I always have a drop of a second on my 5/3 fps and Minecraft freezes for a second and then starts again at 500 fps why??
 

Steps to Reproduce:
it didn't do it to me before