I'm running a Postgres database with a total disk occupation of 100Gb, largest and most used table up to 40Gb (about 30.000.000 tuples).
Overall performance degrades sometimes due to some queries that are not run by the final user app. I guess they are run by Postgres itself. They use to take up to 100% of CPU and delay user queries substantially.
From 'pg_stat_activity', you can see this pattern in "current_query" column for these queries:
SELECT * FROM "public"."tablename" ORDER BY "column1", "column2"... LIMIT 1000 OFFSET 144000
Sometimes with 'SET DATESTYLE = "ISO"'; before the SELECT.
there's nothing like that run by postgres itself automagically, it must be some software at your end you're not aware of.
in pg_stat_activity, check usename, client_addr and client_port, and match this up against netstat or whatever activity to determine what application is making these queries.