“Indexes achieve this with the additional costs of storage, memory, and keeping it up to date (slower writes), which allows us to skip the tedious task of checking every table row.”
Most application state lives in a relational database, and most developers treat it as a black box behind an ORM. This walks through B-tree indexes, transactions and isolation levels. Every index speeds up reads and slows down writes. Nobody gets that trade for free, and the slow endpoint in production is usually where someone forgot.