N+1 problems were the primary bottleneck in a Websphere backendi worked on for years.

Transactions were being aggregated with thousands to 10s of thousands of SQL statements like this.

    SELECT * from customer
    where id = <single id>

The developers had no idea the ORM was doing this under the hood.

It's trivially easy to say that if every SQL statement took 1ms to execute, that thousands of round trips can really start to tank whatever process it might be blocking.