Why are NUnit testing times slow when using 'Run All' in VS 2015 Test Explorer?

I am using Test Explorer in Visual Studio 2015 via the NUnit VS Adapter to run my unit tests.

When using Test Explorer's 'Run All' command my tests run and show pass/fail within a second, yet the total runtime is 34 seconds.

When selecting all of the tests and using 'run selected tests' from the right-click context menu the same tests take a total runtime of 1 second.

I've have not found any clues as to why it takes so much longer to use 'Run All'.