Do not let Drupal handle 404's on development environment
Newer versions of Drupal have an addition to settings.php so that it doesn't handle the 404 for certain assets:

$conf['404_fast_paths_exclude'] = '/\/(?:styles)\//';
$conf['404_fast_paths'] = '/\.(?:txt|png|gif|jpeg|css|js|ico|swf|flv|cgi|bat|pl|dll|exe|asp)$/i';
$conf['404_fast_html'] = '&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;&lt;head&gt;&lt;title&gt;404 Not Found&lt;/title&gt;&lt;/head&gt;&lt;body&gt;&lt;h1&gt;Not Found&lt;/h1&gt;&lt;p&gt;The requested URL "@path" was not found on this server.&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;';


There are comments in the file that explain these new options some more.

You can also edit the .htaccess to disable the ErrorDocument and then put some ErrorCond directives in to prevent missing files from being punted to index.php:



RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !=/favicon.ico
RewriteCond %{REQUEST_URI} !files/foo/(.*)\.bar$
RewriteRule ^ index.php [L]