Ofcourse, everyone is entitled to have their own definition of sluggish. However, it can easily be determined in an objective manner that iOS 9 uses less frames to render the same animations with the same duration as iOS 8, which results in an apparent reduced smoothness.
The smooth iPhone UI is one of the main reasons is I bought my first iPhone back in February 2008. So yes, it bothers me. I don't want my $1000 iPhone (most expensive phone in the world) to feel like a $500 Android, and right now it does.