What causes Postgresql performance sag?

Question

PostgreSQL

There is a Postgresql database with approximately the following functionality:
table1 - approximately 3 million records, six indexes, one of them is clustered. The data in it does not change, it is only searched.
table2 - changeable table, active insert/update goes to it. New records are looked up in table1.
Speed ​​of work:

As you can see, the first portions are processed at an acceptable speed, and then there is a drop in performance (arrow 1). Moreover, this fall occurs at a random moment, not necessarily after two servings. If you do a vacuum analyze on table1 (which is not changeable), then the performance increases again (arrow 2).
Tried to disable automatic garbage collection on table2 - nothing has changed.
The problem is reproduced on two different computers - under win7 and win10. PostgreSQL 9.5.

Answer 1

Where exactly is your performance gap?
0.003 ... 0.015 ... 0.012
means nothing. If it were 0.015 everywhere, and suddenly 1.15, then you can already start looking for the cause.