Meaning a miner would need to wait for 5 minutes of sequential calculations before being able to put all the parallel computation power to work.
So for a block time of 10 minutes, there would be 5 minutes of low-energy-demand VDF calculation and 5 minutes of high-energy-demand parallel hashing (current PoW).
That would cut the total energy consumption roughly in half, and this could be scaled up to 9 minutes of VDF calculations or more, to reduce the energy footprint even further.
Is this viable?
1. You are trying to address a non-problem (the "bitcoin uses too much energy" drama is based on a lie)
2. Since you cannot force the miner do this or that, imho this cannot work. I mean, let's say they gracefully ignore your flag; what will you do then, sue them?