PostgresSQL 9.5.21 very slow to connect and perform basic queries

I'm investigating an issue on a PostgresSql 9.5.21 installation that becomes unusable in an intermittent way. Simple queries like "select now();" could take 20s. commits take 2s. and all gets fixed after an engine restart.

I look into the pg logs and no signs of errors. and checkpoints are always timed. The machine is well provisioned, load isn't too high, and cpu io wait is under 1%.

any suggestions on what I should check more?

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