Having to constantly look up and parse rules (especially spells) are what lead to cognitive overload for me. I'm already concentrating so much on the players and reacting to what they are doing, it is too much for me to shift gears and start looking at books and notes in a detailed way. Ideally I would either just remember, be able to figure out how to adjudicate it ("roll intelligence check"), or it would maybe be on a rules reference no longer than 4 pages.C. Mental Overhead for the DM. This is often-overlooked, but for DMs (especially new DMs), it is often beneficial to be able to rely on a rule. For most people, it is less mentally taxing to be able to just use a rule than to come up with an ad hoc adjudication. There is an AC, there is a to hit roll, boom.
Same process really if it is TotM: DM decides.Another reason I prefer TotM.