Even assuming a perfect computer with zero input latency and zero display latency the network part still matters when it comes to knitting an area like "North America" into a game region. It feeds into longer causal chains.
For example, consider two players in LA both using an NY server over the dedicated zero-overhead fiber-optic network mentioned above, with a relatively modern game-networking stack.
(1) Player A raises an energy-shield t=0 ms. (Client-prediction.)
(2) The server agrees as of t=20.
(3) Player B shot an instant-travel hitscan weapon at t=39, while the victim was still exposed on his screen.
(4) The server gets the shot-message at t=59, and honors it (Latency Compensation), sending a damage/death message out to Player A.
(5) Player A receives the news of his damage/death at t=79.
Even in that world of unattainably-good equipment, that's 80ms of "wait, that doesn't look right".
