Yes, i did felt the same but with later updates i felt the device became sluggish.
Not because of S8, it is superior device everyone knows with better hardware specs but S7 has much dated hardware in current date, would not recommand anyone to buy the device as its costly and more importantly old now.
Yes, i thought the same and tried too but it didn't worked and still same issues. Keyboard takes 3 seconds atleast to open. Contacts menu open after 1 second. And so on...
Marshmallow was surely bloaty and heavy but since it came preloaded with the device, it was better optimised for the device and made for the device. The Nougat update is may need better hardware to run smoothly, like S8 has.