you use struts2 and java technologies.... then it will be very fast... but developing takes lots of time...
Host your image, css, JavaScript files on a Content Delivery Network e.g. http://www.maxcdn.com/ or google "Amazon CDN". Because they have servers located around the world, typically your website may be suited to this therefore quicker for visitors/users closer to server. Larger companies tend to be more reliable.
use cache and big commerce websites doesn't open good in shared servers. You need to upgrade to either VPS or Dedicated one. Magento is good ecommerce opensource. Also you need to check code source, css and javascript coding pattern.
I would recommend the Turpentine extension from Nexcess. They also have an outstanding Whitepaper that presents the best practice for Magento performance. You should definitely check that out. http://www.nexcess.net/magento-best-practices-whitepaper
Check out the "Methods To Speed Up Magento" at http://www.magentocommerce.com/boards/viewthread/36225/ Hope this make sense.
I've been working with Magento for good 3-4 yrs now. Firstly, its a framework best suited for dedicated servers which can be fine tuned for better delivery. There are some optimized hosting solutions out there available for Magento as well. You might want to check up on those too. Secondly, make sure your compressions are enabled in Magento. You can also generate free performance reports for your website and see if there are any hold ups there as well.
Here is the answer for improving your site loading speed. http://www.wd3.com.au/making-your-magento-ecommerce-site-perform-fast/