One cannot compare constructing a brand new airport on a greenfield site to expanding somewhere like LHR.

If LHR itself, the M4, and the M25 all were to close and be dedicated to construction traffic only, it would all be completed far more promptly.

The tricky thing is often maintaining operations at a site while at the same time building and expanding the same.

Witness the relative ease of building T5 to the complexities of T2.