How to increase the speed of exposed autocomplete field in views using drupal 6?
Assuming a Drupal installation configured for a production environment (including Apache and MySQL tuning), there are two things that impact autocomplete latency.

One, is a built-in delay between user input and running searches.  This is baked into misc/autocomplete.js.  The value is set to 300ms (line 245), and get used in Drupal.ACDB.prototype.search (line 287).

The second is that searches will run on a single character.  If your autocomplete result set is large, this can have huge performance impacts.

Unfortunately, there isn't a good way to alter either of these behaviors.  You can add your own Javascript and redeclare Drupal.ACDB to use a shorter delay, eg

Drupal.ACDB = function (uri) {
  this.uri = uri;
  this.delay = 100;
  this.cache = {};
};


and Drupal.ACDB.prototype.search to add in some logic to enforce a minimum number of characters, eg

...
// See if this string needs to be searched for anyway.
searchString = searchString.replace(/^\s+|\s+$/, '');
if (searchString.length &lt;= min ||
  searchString.charAt(searchString.length - 1) == ',') {
  return;
}
...


However, 400 results isn't a huge number.  I have mainly seen the delays this with 10000+ results.  This suggests that you may need to do some additional profiling on your server.