Initially I would be tempted to define the maximum operating frequency by the critical path, as being the propagation time of each one of the three NOR gates, added to the propagation time of the AND gate.
However, as there is a symmetry in the circuit, in theory one side should balance the delay of the other, but I believe that the maximum frequency is effectively determined by the asymmetry due to the real world different characteristics of each component, just a guess.