Diagnosing poor insert performance in production?

Where can I look to if I'm trying to diagnose poor insert performance?

The database is behind a pgbouncer instance receiving on average ~1100
requests per second. It's read heavy, but has the occasional insert. I'd
estimate about 20-50 per minute.

In the past few days, we loaded a large amount of rows into the database
(about 5 million on one table, 5 million on another, etc.) and now insert
performance is poor on all tables... even tables that didn't get data loaded
into them.

I thought insert performance only went down when you try to concurrently
insert a lot of data, because each insert has to wait on the other in the
case of an auto-incrementing index. So I'm kind of lost here.... does anyone
have any clues of what I should be looking at?

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