I know the official word is that the 5s does not get throttled, but I noticed a huge difference between the way my 5s worked on iOS 10 (great) vs iOS 11 (terrible). Maybe that's simply due to more demanding software.
After a couple of days thinking I'd buy current-gen SE if a new one isn't released on Tuesday, I'm back to thinking I might just paying for a battery replacement. I just don't rely on my phone for much more than its camera, music and podcasts. The 5s handles all of those fairly well, even if it's slower than ever.