Updated my spare 5S to iOS 12 via iTunes ytd.
First impressions is that it is DEFINITELY faster than iOS 11.4. Opening and closing apps is more responsive and has less lag. Camera and most Apple apps open a tad faster, which contributes to a more positive experience than 11.4.
3rd party apps, however, still open as slow as 11.4, though I suppose that is not Apple's fault.
Would recommend to update!