Drop in performance for each INSERT/DELETE combo

We're using a function to insert some information into the database.
This information is later (within seconds) retrieved from a program,
that does the actual processing of the information. It is then
deleted from the database when we're done with it.

We see a MAJOR performance loss the longer the time. It starts out
from around 28 'data chunks' per second (inserts in a couple tables),
and drops down to below 10/s...

If doing 'VACUUM ANALYZE' every 20 minutes improves the performance,
with the expected drop when the VACUUM is done, but in general the
performance is steady...

Investigation have shown that it's the actual DELETE that's slow,
any idea how to find WHERE (and hopefully WHY :) this is so?
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