I've revised my opinion of why Monaco is so fast, though a springy track may still have something to do with it. It apparently is a no-wind venue.
From the top on the google maps one can see it's roofed over with wind only able to get in from the southwest:
From the weather underground we see the prevailing wind in Monaco this time of year typically comes from any direction but southwest, with evening winds tending to be east to southeast.
of course that southeast wind going past the southwest entrance, with no corresponding entrance on the northeast end, may still infiltrate and blow around in the stadium somehow. But I bet it's either relatively still, or even engineered to create a counter-clockwise vortex in there. Giving the runners a tailwind the whole way around would be both legal and a massive performance boost.