Why there is performance differen between lumia 950s bought from the same source?

Hello guys, several months ago, I bought 2 lumia 950s for me and my brother, directly out of the box, we noticed that my brother's phone was a bit laggy and slow while mine was working brilliant. although my brother's internal memory is less busy than mine. why there is this difference and is there a way to fix the issue?
Thank you in advance

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