Are you reaching the limit of the write capacity of a single database server? (If read capacity is your problem, you should set up master-slave replication. Also make sure that you have first given your database the fattest hardware you can afford, you have optimised your queries, and your schema cannot easily be split into shards.)

Is your amount of data greater than a single server can sensibly hold?

Are your page loads being slowed down unacceptably by background batch processes overwhelming the database?