The test Lucene.Net.Tests.Index.TestAddIndexes::TestAddIndexesWithCloseNoWait() (in Lucene.Net.Tests._E-I) is usually (around 90% of the time) correctly completing in about 2 seconds. However, occasionally it can take much longer. The test only checks for an invalid condition (adding index to a disposed IndexWriter) and is supposed to exit early because of the exception. However, there seems to be contention between the threads that makes it extremely slow to exit on some runs (sometimes up to 5 minutes or more).

No doubt, this is one of the primary contributors to the tests in the CI environment taking excessive time to complete.