  We study the time evolution of the excess value of capacity of entanglement
between a locally excited state and ground state in free, massless fermionic
theory and free Yang-Mills theory in four spacetime dimensions. Capacity has
non-trivial time evolution and is sensitive to the partial entanglement
structure, and shows a universal peak at early times. We define a quantity, the
normalized "Page time", which measures the timescale when capacity reaches its
peak. This quantity turns out to be a characteristic property of the inserted
operator. This firmly establishes capacity as a valuable measure of
entanglement structure of an operator, especially at early times similar in
spirit to the Renyi entropies at late times. Interestingly, the time evolution
of capacity closely resembles its evolution in microcanonical and canonical
ensemble of the replica wormhole model in the context of the black hole
information paradox.
