If it was major upgrade you may need to do a factory reset.
But first make sure to backup everything: pictures, videos, contacts. And maybe messages, too.
Welcome to Android Central! If you're talking about the S7, there have been a number of complaints about the Oreo update. Try wiping the cache partition: https://support.t-mobile.com/docs/DOC-28028.
Remember, Samsung is not synonymous with Android. I've used Pixel/Nexus phones for years, and have never had problems with slowdowns after a major system update. You don't have to go to an iPhone just because of problems with one Android phone.