Remember that Harry had also learned that Quirrell had successfully used Avada Kedavra on two Death Eaters. Moody says that it isn’t hard to cast AK for a second time, and Harry already knows that this time would have been at least Quirrell’s third.
I interpreted the ease of casting the spell as a specific application of scope insensitivity rather than a change in the requirement to cast it. That is, while casting it the second time might be just as difficult (i.e. take as much mental/magical/spiritual energy) as the first, the third and fourth time would together be only as strenuous as the first, as would the collective fifth through eighth time, etc. It is already established in-universe that some form of personal mana depletion exists, and my idea of this difficulty reduction is an extension of that form of energy to the spiritual energy (established in canon w.r.t. horcruxes, dementors, etc.).
Remember that Harry had also learned that Quirrell had successfully used Avada Kedavra on two Death Eaters. Moody says that it isn’t hard to cast AK for a second time, and Harry already knows that this time would have been at least Quirrell’s third.
I interpreted the ease of casting the spell as a specific application of scope insensitivity rather than a change in the requirement to cast it. That is, while casting it the second time might be just as difficult (i.e. take as much mental/magical/spiritual energy) as the first, the third and fourth time would together be only as strenuous as the first, as would the collective fifth through eighth time, etc. It is already established in-universe that some form of personal mana depletion exists, and my idea of this difficulty reduction is an extension of that form of energy to the spiritual energy (established in canon w.r.t. horcruxes, dementors, etc.).