There have been reports of benchmarks showing the 5S being a bit (tenths if a second) slower on iOS 11.
I’ve been beta testing 11 on a 6S, 6, 5S & iPad Air 2.
TBH the 6 and 6S units were more unstable prior to the Golden Master and the 5S seemingly ran perfectly with no noticeable change in performance. The 6 has the most drivability problems but these disappeared with the final beta a week ago.
Even now I have a bit of (non frustrating) animation stuttering (keyboard when typing) on my 6S that has not been present on the 5S. (I recall similar from previous annual upgrades and Apple always fixed that. This year it’s almost unnoticeable so don’t fret.)
I recommend all 5S users (as well as all newer units do the iOS 11 upgrade. There are also security fixes and a bunch of improvements in Control Center as well as a nice new document scanner & data table option built into notes).
Stability and speed seem acceptable on all the above models.