We are sorry to hear that is happening but we want to find an answer for you. Please let us know some more about what you are experiencing. What exactly happens to your device and are you doing anything specific when it happens? The more information that you can provide, the easier it will be for us to come up with a solution. We may serve you better/speedily, if you can provide logs by registering Error Section (Samsung Members App -> Feedback -> Questions/Errors -> Create -> Error reports).
you may go to the service center and explain your problem to them they will fix it and there is no gallery lag in nougat update it might some error on your phone there is no need of reinstalling marshmallow. reinstall nogat once again and the problem will be fixed meanwhile use Google photos it has got many other great features than Samsung gallery app.
