Writing to a drupal table form javascript as fast as possible
If you have zero concerns about user verification and data validation (seems to be the case given the example code) then I would post to a custom script that just boots up the database and does the write. Note that drupal_write_record is only available at the full bootstrap level. 

Example code below:

/**
* Root directory of Drupal installation.
*/
define('DRUPAL_ROOT', getcwd());

// Boot up Drupal.
require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);

$name = $_POST['propertyName'];
$value = $_POST['propertyValue'];
$result = db_query('SELECT * FROM {global_properties} WHERE property=:property', array(':property' =&gt; $name))-&gt;fetchAll();
$record = array(
  "property" =&gt; $name,
  "value" =&gt; $value
  );
$primary_key = !empty($result) ? array('property') : NULL;

// Load up code for drupal_write_record().
// Untested and might break stuff as we are not at a full bootstrap.
require_once DRUPAL_ROOT . '/includes/common.inc';
drupal_write_record('global_properties', $record, $primary_key);


If you do care about user verification and server side data validation and thus need a full drupal bootstrap but the client does not care about what is returned then httprl's httprl_background_processing() function might do what you need. Example of this workflow below.

/**
* Root directory of Drupal installation.
*/
define('DRUPAL_ROOT', getcwd());

// Cut The Connection.
require_once DRUPAL_ROOT . '/sites/all/modules/httprl/httprl.module';
httprl_background_processing();

// Fully Boot Up Drupal.
require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

// Do various user checks here.

// Run Example code.
$name = $_POST['propertyName'];
$value = $_POST['propertyValue'];
$result = db_query('SELECT * FROM {global_properties} WHERE property=:property', array(':property' =&gt; $name))-&gt;fetchAll();
$record = array(
  "property" =&gt; $name,
  "value" =&gt; $value
  );
$primary_key = !empty($result) ? array('property') : NULL;

drupal_write_record('global_properties', $record, $primary_key);


What the above example will do is return ASAP to Apache but drupal/php will run in the background. If the above example doesn't work there is one other option using HTTPRL in order to get a background worker running.