Is your Galaxy S7 running slow after 11/2018 T-Mobile update?
Here is what I just tried to do and it seems to have worked for now. Be careful when doing this as this may lead to the loss of data.

Installed the app "X-plore". Thanks to the developers of this app - a really great tool! The feature to serve the phone's files over HTTP to the desktop PC is a brilliant idea.

It shows most of the system memory contents. I noticed that directory /sdcard/log was bloated: it had way too many files. Seemed like crash dump files that were zipped into ~6Mb packages. There were hundreds of them and they had dates in the names matching the time when the slowness started.

Deleting those files has freed up ~10Gb of space. The phone is running much better.