[GoblinSlayer]

Foreign keys are slow on delete, not on read. If you have a popular table, say, users, and all other tables refer to it, then deleting a user locks the database for time proportional to the number of foreign keys - good old linear scaling.

[bhaak]

It doesn't lock the database, it only locks the rows of the tables that have foreign keys to the popular tables and are referencing the user you are deleting.

Rightly so because when deleting the user the database needs to do work to keep the referential integrity. Either it nulls the user_id, delete the rows, or it throws an error.

[ch4s3]

Sorry I should have specified read/write and side that you can read from something in memory that is consistent after some short delay.