The US controllers must be doing something right. According to ACI, In terms of operations in the preceeding 12 months leading up to March,03, 9 of the 10 busiest airports in the world are in the US. Of the top 30, 24 are in the US. The only non US airport to crack the top 10? sorry its in France, CDG at number 6. LHR comes in at number 13. VNY, a general aviation airport north of LAX, with no airline operations is a touch busier than LHR. In terms of passenger operations, yeah LHR is pretty busy. That speaks to the capabilites of the airport design and its landside operations abilities however.
Source: Airports council information.
http://www.airports.org