I have a valid US Visa. I will be reaching Terminal 5 of ORD Chicago Airport at 20:00 from Heathrow (London) on American Airlines. And I am having a transit time of 2 hours, and I have to board a flight of American Airlines to St.Louis at Terminal 3. Will that 2 hours of time is sufficient for customs, Immigration check and boarding the flight?
Normally if you have a transit time this short you can ask the airline personnel to assist in expediting the matters. However, CBP had created a page with a report of the immigration wait times at various airports so you can get an idea of how long it might take you to pass through the immigration. The max waiting time indicated for immigration was over 2 hours for July 1st and 7th with an average waiting time for the same days and times being about 30 minutes.
So it's your call.