Speed is imperative : At the expense of speed, PostgreSQL was designed with extensibility and compatibility in mind. If your project requires the fastest read operations possible, PostgreSQL may not be the best choice of DBMS.
Simple setups : Because of its large feature set and strong adherence to standard SQL, Postgres can be overkill for simple database setups. For read-heavy operations where speed is required, MySQL is typically a more practical choice.
Complex replication : Although PostgreSQL does provide strong support for replication, it’s still a relatively new feature and some configurations — like a primary-primary architecture — are only possible with extensions. Replication is a more mature feature on MySQL and many users see MySQL’s replication to be easier to implement, particularly for those who lack the requisite database and system administration experience.
