Very slow NUnit tests reordering phase

Hello, we use TeamCity 2018.1 (build 58245).

There is a project on which TeamCity seems to spend a lot of time (16 minutes) reordering tests before running them.

There are two test assemblies which have 20,700 tests combined.

 [10:04:47] Step 6/6: NUnit (22m:40s)
[10:04:48] [Step 6/6] Starting: C:\BuildAgent_2\plugins\dotnetPlugin\bin\JetBrains.BuildServer.NUnitLauncher.exe #TeamCityImplicit
[10:04:48] [Step 6/6] in directory: C:\BuildAgent_2\work\8a4485f20ad4a8b3
[10:05:02] [Step 6/6] Start TeamCity NUnit Test Runner
[10:05:02] [Step 6/6] Running NUnit-2.6.4 tests under .NET Framework v4.0 x64
[10:05:02] [Step 6/6] TeamCity Test Reordering. Phase 1: Running selected tests.
[10:21:35] [Step 6/6] Server.Tests.dll (1m:32s)
[10:23:24] [Step 6/6] Server.UnitTests.dll (running for 4m:03s)

How to get rid of the delay?

Thank you.

Best regards,
Stanislav K