Hi,
There is nothing can be done to increase the speed.
It is jQuery AJAX and AJAX takes time. It is directly related to your hosting and respond time. Normally it should get loaded in 2-3 seconds (maximum), which is also same in our DEMO.
To eliminate that loading time we have developed a new shopping cart module:
flexiblewebdesign.com/Virtuemart-Extensions/Virtuemart-Flexible-Drop-Down-Shopping-Cart
It doesn't use the AJAX each time yo trigger the shopping cart page, so it is relatively faster and user friendly.
You may consider to purchase this module instead. It is a standalone module (doesn't require PureMArt pre-installed) and it costs $10
You may also check for an alternative "free" drop down shopping cart module. (try
www.google.com to search for free alternatives)
Regards.
Support & Development