Description

When trying to export a form with a large number of submissions to Excel or XML in Form Reports, the task might take a long time to be performed. Export might also fail without any exported file.
The problem also might result in high memory load and cause the server and the site to slow down. 
The issue affects WFFM 2.3 - 9.0 Update-2 versions.

Solution

To resolve the issue, consider one of the following options:

• For WFFM 2.3 – WFFM 9.0 Update-2 download and install the patch compatible with the affected product version found on this page:
  https://github.com/SitecoreSupport/Sitecore.Support.66965/releases.
  
• To get a solution for other WFFM releases, contact Sitecore Support.