Is there a reason for not supporting PDO::ATTR_PERSISTENT when calling PDO::__construct? I guess this will affect performance only if DB is not @localhost because TCP handshaking adds extra latency. On the other hand, too many idle persistent connections to a DB may hit some limit in the DB. It's best to have ability to configure this in settings.php.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
shows 75 requests/sec, with persistent connections enabled it shows 90 requests/sec
What Drupal does should be good enough
These two statements contradicts each other.
PostreSQL connection is relatively slow especially when SSL is enabled (that's done by default in many distributions). When page cache is enabled it may happened that ~80% of request time for anonymous visitors is spent on establishing DB connection.
I think it worth reopening.