Freshly installed 4.3.0 installed as CGI on IIS5. Minimal changes to default php.ini

I use the following page to demonstrate/test the problem:

<?
if ( isset( $_REQUEST['test']) ) {
  $ttt = $_REQUEST['test'];
} else {
?>
<form action=test.php method=post>
<textarea name=test></textarea>
<input type=submit>
</form>
<?
}
?>

Very simple post test form which assigns post data to a variable. This particular script functions correctly with small amounts of data but above around 500 characters (seems to change depending on number of fields in form) the page will sit there for about 5 minutes until i get a CGI Timeout error from IIS (also logged to event viewer).

Items from phpinfo() relating to post are set to defaults:
  always_populate_raw_post_data Off
  post_max_size 8M

A similar ASP page on the same web server functions perfectly up to thousands of characters.
As far as I am aware all other PHP functions are working correctly.