We will fly in and out of London Heathrow for our May vacation. We're looking at early morning arrivals (6:30)and noon (12-1) departures. Are there any arrival/ departure time frames I should to consider when terminals 3,4,5 are absolutely jammed vs smooth vs clear?
I think you'll be fine with that time frame. In our experience, immigration/ passport control lines (arrival) have been longest mid-day. Our departures are usually 11ish. We make sure we leave enough time to get through security ( and they take that 1 qt. bag rule seriously). The terminal is usually very busy, but they have larger waiting areas as gates aren't assigned as quickly as they are in the states.
Heathrow is always busy! However, a lunchtime departure can help to miss the worst traffic (if you are catching a cab to the airport) or missing the worst crowding on the tube.
Early mornings are always busy for arrivals as many flights arrive after the night curfew ends. Having said this, I have never found it to be an issue. I have experienced much worse delays at US airports such as O'Hare!
Also for departures, remember you should be at the airport 3 hours before your flight takeoff time. I'm looking at a noon departure from LHR to STL in early November, which I think is a good time because I can be at the airport at the reasonable morning hour of 9:00.