My take is the opposite, I feel it is definitely faster than OS 4!
It is significant enough to be really noticeable. To me both iOS 12 and watchOS 5 are stable and offers a speed increase, more so on the watch than the phone.
I feel my watch is a bit faster on OS 5. A bit faster on opening apps, changing complications/faces and when I open the Music app on my phone, I can see the Now Playing on my watch. Before on the OS 4, it sometimes take a while before the Now Playing will show the song currently playing.