Other databases are forgiving; they are configured "safe", even at the expense of speed. The intention is that you can deploy a small system immediately; if/as you grow you will see that the database is going too slowly. You can _then_ look at the performance/safety dials you can tune and choose appropriate trade-offs.

These are systems designed for the real world, where people don't read the manual until they have to.

When people assume MongoDB was similarly designed with their best interests in mind, that's when things go wrong.