Significant test execution time increase after migrating from SpecFlow to Reqnroll (~19k tests)

[krishnaduvey]

Summary
After migrating our BDD test suite from SpecFlow to Reqnroll, total execution time increased significantly.
This is currently blocking stakeholder acceptance.

Scale
• Total tests: ~19,000
• Execution model: sequential in distributed environment

Environment
• Reqnroll: 3.3.4
• Reqnroll.NUnit: 3.3.4
• .NET targets: net10.0, netstandard2.0
• Visual Studio Community 2026 (18.6.2)

Problem statement
For the same test estate and comparable infrastructure conditions, execution time after migration is materially higher than before migration.
We are looking for guidance on:

1. Known causes of runtime increase in large migrated suites
2. Supported configuration options to reduce execution overhead
3. Recommended migration-time performance tuning checklist for large suites (~19k tests)
4. Any known regressions in Reqnroll 3.3.x affecting large test inventories

Expected behavior
Execution time should remain in the same operational range as pre-migration baseline, or there should be clear documented mitigations for large suites.

Actual behavior
Execution time increased enough to be unacceptable for current release/stakeholder targets.

Request,
Please advise:
• What diagnostic data you want from us first (logs, timing breakdown, sampling strategy)
• Which configuration/runtime settings should be validated immediately for large suites
• Whether this matches any known issue already tracked

[304NotModified]

What was the SpecFlow environment? Also nunit, net 8?

What is significantly? 10% more? 2 times? Anf is it seconds? Minutes? Hours?