After getting updated to Oreo a few weeks ago, both S7s in our household are suffering from very poor battery life. Checking battery use it is Android System using 20% and Android OS using 10%. Everything else is 1% or lower. Even with no phone use, the battery drops like a stone.
Also, the phones are very slow - normal use like browsing, switching between apps, opening new apps - it generally feels much slower than before the update.