Izzet Delver is a fast tempo deck relying on free countermagic (Force of Will, Daze) to protect Dragon's Rage Channeler and Murktide Regent, so revealing Chancellor of the Annex from our opening hand taxes their very first spell, which can blunt their explosive draws before we're even on the play. Later in the game its flying 5/6 body and ongoing tax effect continues to slow their cantrips and threats. We cut Throes of Chaos since it's our least mana-efficient cascade spell and this matchup rewards resilience against countermagic over raw card selection variance. Ancient Tomb and Boseiju stay in as-is since our manabase needs to keep pace with their fast clock.