Combine Webforms to export multiple form results in a single CSV
Webform 4.x is integrated out of the box with views on drupal 7.


Upgrade to last webform 7.x-4.x release.
Create a view: webform submissions 
Select each field: webform submission data:Value and your webform node to extract the fields 


To download all results the best option is Views data export, you should configure it to use batch for better performance.