How to fix S7 edge performance degrade after Oreo update? My S7 edge (G9350) upgraded to Oreo for a month and the performance really degraded. I did factory reset to my S7 but  nothing improved.

I want to flash my S7 edge G9350 from Oreo to Nougat/Marshmallow but did not succeed  because of FRP .I don't understand why I got locked by FRP even  when I entered correct Google account ( now my S7 working is functional except the performance )

Are there any possible ways to flash my ROM to Marshmallow ?
There is no way to downgrade as bootlader is now on revision 7 (if I recall correctly) and Marshmallow/Nougat uses way older ones. So an official update won't be an option.

The other method is to use a custom ROM from XDA (you'll find many there), but be advised that that'll void your warranty and could brick your device (tho unlikely).

If you want to use a custom ROM, you can either go for an old TouchWiz ROM or (like I did on my S7) use an AOSP-based ROM. Presonaly I use LineageOS 16.0(9.0), but 15.1 (8.1) is preferred for stability as 16.0 is still beta. I'd recommend you use a custom AOSP ROM over an old TouchWiz ROM as those old ROMs don't have security patches and using an old Android version is never a good idea, not only because of security, but also you're just downgrading your phone in terms of features.