The Prius ICE during warm-up will essentially try to pollute as little as possible at the expense of fuel efficiency, whilst reducing the warm-up time as much as possible.
In this PriusChat post an interesting and extremely detailed overview of the warm-up stages.
A graphical representation is available on PriusChat here .
It is not yet clear whether there are differences in the warm-up stages between the US, EU and Japanese model. Moreover, it is not yet fully clear whether during warm-up the ICE is absolutely not pushing the car directly (with torque via MG2) or indirectly (via electricity generated from MG1), or simply energy is taken from the HV Battery only. In the latter case, to reduce fuel consumption, it has been advised by several PriusChat members to keep the car still during at least the first 50 seconds (stage S1a), or at least, if pollution regulations do not allow it, to drive with the least minimum speed, to preserve battery power.
It is also not yet confirmed with repeatable data, whether this will affect overall consumption, since the average consumption is dependent on several factors (some of which affect the warm-up stages as well, such as external temperature) and the overall length and type of trip.