No they don’t slow them down, but iOS 11 runs like utter dog **** on my 9.7” Pro, I mean really rubbish. I’ll have to do a complete wipe and reinstall at some point.
Did you OTA update? Don't do that with major versions of iOS (eg. iOS 10 --> iOS 11).
Always back up to iTunes and restore, then restore backup, if you want good performance.