Just run this command in the terminal to turn off Indexing:
defaults write com.apple.dt.XCode IDEIndexDisable 1
To turn it back on, run this:
defaults write com.apple.dt.XCode IDEIndexDisable 0
(Note: Apparently you need to delete this key in order for the change to take affect, however, I used simply the above command and it worked fine. So if doing the above doesn’t work, try deleting the key)
EDIT
Sorry, missed part of the question. Yes, it will make it so searching does not work as fast. Perhaps auto-complete will get disabled. Indexing is what allows Xcode to quickly remember what you have done. Turning it off will make it slightly harder to work with, but it improves loading time.