This is the RCS function of Android. You can use Wi-fi or mobile data to send pictures and texts, sort of like WhatsApp.
Its a great function when chatting to other RCS users.
As for a slow device highly recommend 'Good Guardians' from the Galaxy store. Great way to keep the performance of your device at its best as well as monitoring functions for battery, memory, etc.
It's natural for devices to slow down for a period after an update. Good Guardians counters that.