When I load the game and select a button the first click is quite laggy and not sure why, after that touch its smooth and works fine
Due to the amount of information you provided is hard to tell any relevant answer for that.
Although, maybe it helps you out, if you check out the Unity’s Profiler and how it works.
It will tell you exactly which method causes the trouble when you profiling your application during Play mode. (If not switch to Deep Profile, it will tell you more information, but it may have additional performance impacts)