Slow views render time
Though view is a powerful module, it can cause a lot of performance problems if certain things are not handled. There a certain steps you can follow to fix this.

There are several things you can do to solve improve this:  

Optimizing View:


Try to change left join to inner join using alter. 
Enable Cache on views and clear it when order is updated.
Check if Distinct is marked.


Here is a sample views alter code:

/**
* Implements hook_views_query_alter().
*
* This disables distinct and turns LEFT joins into INNER joins. This increases
* performance.
*/
function my_module_views_query_alter(&amp;$view, &amp;$query) {
if ($view-&gt;name == "my_view") {
$query-&gt;table_queue['content_type_article']['join']-&gt;type = 'INNER';
$query-&gt;distinct = 0;
}
}


Optimizing view is a wider topic, there is a good write up on optimizing views that you can read to understand this better here.

Other than this you should check if your mysql configuration in my.cnf is optimized as per your system resources. For this you could use mysqltuner script or mysql-tuning-primer script.

Above that use of memcached with Drupal can also increase performance to great extend.