Repro steps:

1. Open provided project in 5.6.x
2. Click on "Window" -> "Test Runner" ("Editor Tests Runner" for 5.5.x)
3. Click on "Run All" and measure the time it takes for the tests to finish
4. Repeat for 5.5.x

Expected result: The speed at which the tests run should be approximately the same for both of the versions
Actual result: In 5.6.x the speed at which the tests are run is significantly slower than in 5.5.x

Fixed in: 2018.1.0b1

Reproduced with: 5.6.1p1. 2017.1.0b6, 2017.2.0a1
Did not reproduce on: 5.5.3p4

Note:
All of the tests are like this: "[Test] public void Test001() { Assert.IsTrue(true); }"
In version 2017.x it's even slower
Added videos of all 3 major versions