I am having a similar problem with my iphone 4s. Spotlight Search has become incredibly slow, when selecting it via the home button or swiping (to the left) to the search screen. After I reboot my phone, the first couple swipes will be relatively smooth, but it is all downhill from there.
First, I experience lagginess as the screen is switching from 'home screen' to 'search screen'. It essentially looks like it is stuck between the two screens (typical lag characteristic). It will remain frozen like this for multiple seconds. Then, when I try to type something, the keyboard immediately lags and it takes a minimum of five seconds for the would-be text to appear in the search bar.
I have tried deselecting ALL (as well as only mail, music, etc) 'search catagories' under 'Spotlight Search' in settings followed by a reboot- to no avail. I also tried syncing my phone with itunes, and deleting my cloud backup, but the lagginess is still there...
The strangest part is I did not experience this problem until approximately one week ago. I could swipe back and forth between the 'home' and 'search' screen endlessly with very minimal lag. Transitioning from one screen to another, typing search queries, and getting search results all previously occurred fluidly. Further, I have not dealt with any other lag issues with the phone. Apps load up quickly, graphics do not seem to be affected either.
Spotlight search is an important, but basic function. This lag renders it useless. Apple needs to get on top of these simple issues before ios shapeshifts into android...