I have never connected at SNN, but it is such a small place that it won't take long at all to get to US immigration - remember there is no immigration or customs to worry about from LHR. However, I believe you will have to reclear security, and you might have to recheck-in at SNN - will you have a boarding card already? In addition, what you might have to worry about is the cutoff/lead time to get through US immigration in SNN. I have never personally experience any delay there, but others have reported long queues at times, and US airlines at SNN all have requirements about having to be in the "sterile" area after US immigration some XX minutes before scheduled departure.

So, place is small, but time might be tight if you have to recheck.