San Francisco - Transfer from International Terminal to Terminal 3?

Hi,
I've found a couple of other threads on this but they don't completely answer my question.
I fly LHR to SFO on Monday and I have a 2 hour transfer window to get to domestic T3. I will only have hand luggage and hold a US passport. Do I have to go through passports and customs, out into the street, walk to T3 and join the security line into that terminal, or is there a shortcut for transit passengers direct from one terminal to the other? Because the flight is from LHR it is bound to be late so concerned that 2 hours isn't enough unless there is a shortcut.
And if the flight is running late, do United staff wait at the gate to whisk me off the the other flight, as I have seen at other airports?
Thanks!

Yes, you do have to go through immigration and customs. You will then be required to exit the secure area. Go upstairs and re-enter through the international terminal G, usually security lines at the international terminal are not too bad at the time when the LHR flight(s) arrive. There is an airside passage to United domestic.

Two hours is plenty of time.

If the flight is running late, there may be United staff at the gate. Unless you are a global services member, they will totally ignore you and refuse to help if you ask. This is the new United, so you should not expect any staff member to care about you regardless of the situation.

Many thanks! I look forward to experiencing United's love first hand...

Just to add in case there was minor confusion. As you arrive landside after customs, you have a choice of using either the international or T3 security lines. International and T3 are connected both land- and airside, so you can pick whichever one is moving faster. Good luck!