Here is a wrapper to log in a file each session's operations. Useful to investigate sessions locks (which prevent PHP to serve simultaneous requests for a same client).
Just change the file name at the end to dump logs where you want. 

class DumpSessionHandler extends SessionHandler {
    private $fich;

    public function __construct($fich) {
        $this->fich = $fich;
    }

    public function close() {
        $this->log('close');
        return parent::close();
    }

    public function create_sid() {
        $this->log('create_sid');
        return parent::create_sid();
    }

    public function destroy($session_id) {
        $this->log('destroy('.$session_id.')');
        return parent::destroy($session_id);
    }

    public function gc($maxlifetime) {
        $this->log('close('.$maxlifetime.')');
        return parent::gc($maxlifetime);
    }

    public function open($save_path, $session_name) {
        $this->log('open('.$save_path.', '.$session_name.')');
        return parent::open($save_path, $session_name);
    }

    public function read($session_id) {
        $this->log('read('.$session_id.')');
        return parent::read($session_id);
    }

    public function write($session_id, $session_data) {
        $this->log('write('.$session_id.', '.$session_data.')');
        return parent::write($session_id, $session_data);
    }

    private function log($action) {
        $base_uri = explode('?', $_SERVER['REQUEST_URI'], 2)[0];
        $hdl = fopen($this->fich, 'a');
        fwrite($hdl, date('Y-m-d h:i:s').' '.$base_uri.' : '.$action."\n");
        fclose($hdl);
    }
}
ini_set('session.save_handler', 'files');
$handler = new DumpSessionHandler('/path/to/dump_sessions.log');
session_set_save_handler($handler, true);