Support PDO::ATTR_PERSISTENT

Actually this is possible

This is how you would do it.

Include this class in your settings.php.

class DatabaseConnection_mysql_persistant extends DatabaseConnection_mysql {
  
  function __construct(array $connection_options = array()) {
    
    //If you unset this, the PDO attribute PDO::ATTR_STATEMENT_CLASS will not be set. Which means you can enable persistant connections
    //These are the comments from the statement class in core

/**
 * Default implementation of DatabaseStatementInterface.
 *
 * PDO allows us to extend the PDOStatement class to provide additional
 * functionality beyond that offered by default. We do need extra
 * functionality. By default, this class is not driver-specific. If a given
 * driver needs to set a custom statement class, it may do so in its
 * constructor.
 *
 * @see http://us.php.net/pdostatement
 */

//Mysql driver does not set this, so we can remove it.

    unset($this->statementClass);
    parent::__construct($connection_options);
  }
  
}

Then in your settings.php

$databases = array (
  'default' =>
  array (
    'default' =>
    array (
      'database' => 'fresh7',
      'username' => 'username',
      'password' => '',
      'host' => 'localhost',
      'port' => '',
      'driver' => 'mysql_persistant',
      'prefix' => '',
      'charset' => 'utf8mb4',
      'pdo' => array(
         PDO::ATTR_PERSISTENT => TRUE,
       ),
    ),
  ),
);

I have not tested this, but in theory it should work

And this seemed to be a duplicate of #843114: DatabaseConnection::__construct() and DatabaseConnection_mysql::__construct() leaks $this (Too many connections)

I'd recommend that folks read What are the disadvantages of using persistent connection in PDO? before attempting this.

What Drupal does should be good enough, as it maintains the same connection for each page request. I wrote a bit more about this over at Connection Pool.

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.