I don't know how much is lost, but there is definitely some. I lose speed more quickly coasting with my hub (Nexus 7) than with a freewheel design. This is confirmed by turning the crank on a tuning stand and seeing how much more quickly the wheel stops turning than with a freewheel.

Similarly, on the tuning stand, if I slowly spin the crank backwards, the wheel will start to role backwards, which never happens with a freewheel (or at least to much less an extent).

Both of these tell me that the hub is creating a bit more internal resistance than the bearings and ratchet mechanism on a freewheel do.

The chart posted above would indicate that internal hubs maybe lose 2.5% of efficiency compared to a derailer.

That and the slightly higher weight are probably why racers all use derailers.

I don't know how much is lost, but there is definitely some. I lose speed more quickly coasting with my hub (Nexus 7) than with a freewheel design. This is confirmed by turning the crank on a tuning stand and seeing how much more quickly the wheel stops turning than with a freewheel.

Similarly, on the tuning stand, if I slowly spin the crank backwards, the wheel will start to rotate backwards which never happens with a freewheel (or at least to much less an extent).

Both of these tell me that the hub is creating a bit more internal resistance than the bearings and ratchet mechanism on a freewheel do.

The chart posted above would indicate that internal hubs maybe lose 2.5% of efficiency compared to a derailleur.

That and the slightly higher weight are probably why racers all use derailleurs.