Steps to reproduce:
1. Open attached project "Test_5.4" with Unity 5.4
2. Open scene "Scene"
3. Go to: Main Menu > BugReport > Reimport Shader
4. Remember the time that have appeared in the Console
5. Open attached project "Test_5.5" with Unity 5.5
6. Open scene "Scene"
7. Go to: Main Menu > BugReport > Reimport Shader
8. Compare two times (5.4 vs 5.5)

Result: on Unity 5.5 Shader import time have increased by ~15%.

Reproduced with: 5.6.0a2. 5.5.0b11
Not reproduced with: 5.4.2p4, 5.3.6p5 (due to errors in the Console)
Regression appeared on: 5.5.0b9 (can't test on earlier 5.5 versions due to errors in the Console)

Note: reproduced on Windows and Mac