I agree with bbrks... it's50/50. It's definitely slower than iOS10.3.3.
However if you disable Siri Search and Look Up suggestions (I had already had to disable Spotlight suggestions in iOS10 to be fair), especially for Safari, Mail and Slack, as well as Handoff, and if you limit background app refresh and "Always" location sharing to a very small and strategic set of apps, you should be doing OK.
iMessage has become really sluggish, though, to the point where I've completely switched to Whatsapp, which is still very fast. I get a lot of reloads and hangs on Safari. Multi-tasking is a bit tough. And Facebook takes ages to load. And sharing photos/videos from the share tab mostly crashes, I'm not sure why.
But I like the butter-smooth opening animations for apps, the new control center, the new Mail design, the absence of text on the icons in the dock, the Files app and it's amazing integration of Box, the better connectivity to public wi-fi, the improved "Memories" tab in Photos, the storage optimizations, better controls over use of location services by apps, always-on Bluetooth for easier use of Airdrop, possibility to shut down Mobile Data in one touch, several levels of intensity with the torch (long-press!), blocking of cross-site tracking in Safari...
So all in all, I think I'm going to remain on iOS11 and hope that 11.x updates increase speed!