validating textfield input with ajax on every key entry
The ajax callback in drupal is nearly as slow as a complete page load and this takes between 200 und 1000 ms. So checking every key stroke is no option.

If you don't check against the database, there is no need for an ajax callback. I would check "right" or "left" in javascript. If you need a server side check, you can write a stand-alone php file, that does this check without drupal, then your response time will be between 10 and 20 ms.

To replace the slow ajax with javascript you can start with this script:

val_left_or_right.js:

(function ($) {
  $('#edit-direction').keyup(function(){
    var direction_text = $('#edit-direction').val();
    if (direction_text != 'left' &amp;&amp; direction_text != 'right') {
      $('#error_message').html('Please type left or right');
    } else {
      $('#error_message').html('OK');
    }
  });
})(jQuery);


attach the js to the form:

$form['#attached']['js'] = array(
  drupal_get_path('module', 'your_module') . '/js/val_left_or_right.js',
);