For simple queries Postgres can do thousands of requests per second on desktop hardware without any serious tuning. Of course if those are complex queries or if they need to handle a lot of data this can be much, much slower. But Postgres is not slow for simple queries.
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In their case, it seems the request in question involves a write to a PostgreSQL database. Depending on the database, 100 can be a lot of work.
They measured that the app spends about half its time waiting for the database.
There seems to be a whole lot of things to do before thinking of changing basic plumbing.
Perhaps then the title of the post could be "handling 100 writes per second with Postgres"
Or perhaps people could consider more than the title when commenting.
For simple queries Postgres can do thousands of requests per second on desktop hardware without any serious tuning. Of course if those are complex queries or if they need to handle a lot of data this can be much, much slower. But Postgres is not slow for simple queries.