Slow in what sense? I was on an iPhone 8 until it broke a few months ago and it was plenty fast.
I had my 6S until last summer (when I upgraded to 11 non-pro, it's the last iphone with a non-flickering screen right now)
The main thing I notice is searching for an app in the search menu takes a long time. Opening apps also takes several seconds, and the phone will often get very hot just from normal use, like the CPU is being pegged just trying to run iOS.
Some have said freeing up space on their iPhone fixes it, but I have not had success with that.
My iPhone 6 is still snappy. Ex has an 8 which is great. How is an X slow?