Defect traceability report stuck on 99% if the report scope is large enough

When generating the defect traceability report, if the report scope is large enough the generation gets stuck at 99% and after a few minutes, the request fails with a 504 error.

Steps to reproduce 

1. Have a large number of test runs, test run iterations and test run iteration steps;
2. Attempt to generate a report without selecting any fix version, test plan or test environment.

Expected Result

The report is generated.

Actual Result

The report generation is stuck at 99% after receiving a 504 error.