What device and OS you on? Can you explain what you see? I occasionally have lag there. When I tap on a contact, it grays a couple seconds before going in. I'm on 6s Plus, iOS 10.1.1.
I have an iPhone X and when I press a person on my list nothing happens for around 5 seconds and then the person comes up. I have done both a hard reset as well as a soft one but that didn't fix it.
iOS 11.2.6 released just recently is supposed to fix this issue. That said if you are not on 11.2.5 then something else is causing it and updating to 11.2.6 will not help. Good luck. Wish the answer was clearer.