Summary

In a project which have thousands of issues and most of them already have different forms attached to them, go to Project settings -> Forms to export all form responses for a particular form. 

If the export only results in a few issues among the large amount of issues in the project, the export should be quick. But in reality it takes a long time.

Workaround

Use issue navigator to list a smaller number of issues and use Export XLSX (form data) to export the form data for those selected issues.