Very long delay when running C# unit test in VS2015

I have a very long delay (about 1 minute! even more) when running unit test in VS2015. When running even a single test, I see the progress bar in the top of the "Test Explorer", it flows for about a minute, and then I get the test result. The reported test runtime is as expected - very small, few milliseconds.

My question is - how can I debug this? How can I see what is going on in this minute before the test actually starts to run?

Thanks.