Lag in the galaxy S7 edge

I bought a used S7 edge, the guy who sold me could not verify the origin of the very cellphone. He had received it as part of a payment someone owed him. The thing is, whenever I used a stock rom the S7edge becomes laggy, if I use any custom rom it works perfectly. I have tried pretty much all types of stock roms and wipes without success. I am no developer so, I can't find any relation between the stock becoming laggy and custom working fine.
Is there a way to fix it?

Thanks

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