Steps to reproduce:
1. Create new Unity project
2. Create new Timeline Asset and select it(New Timeline)
3. Open Window->Timeline Editor
4. Add Track Group
5. Add Activation Track to Track Group
6. Create new Timeline Asset and select it (New Timeline 1)
7. Drag “New Timeline” to “New Timeline 1” Timeline Window
8. Duplicate “New Timeline 1” asset in Project Window (New Timeline 2)
9. Add “New Timeline 2” asset to scene and select it
10. Expand Track Group in Timeline Window
11. Click and drag Timeline Playhead around

Expected results:
Timeline works smoothly
Actual results:
Timeline works noticeably slower than usual

Reproduced in: 2018.1.0a7, 2017.3.0f2, 2017.3.0a2, 2017.2.1f1, 2017.2.0b5, 2017.1.2p4, 2017.1.1f1, 2017.1.0p3
Didn’t reproduce in: 2017.3.0a1, 2017.2.0b4, 2017.1.0p2, 2017.1.0f3, 2017.1.0b1
Regression introduced in: 2017.3.0a2, 2017.2.0b5, 2017.1.0p3