To reproduce:
1. Open attached project
2. Open scene testlag. It contains a lot of gameobjects
3. Open up a few hierarchy windows (or use just 1, it's enough)
4. Select one of the textures from the project view. Notice that it takes a long long time
5. Close ALL hierarchy windows
6. Repeat step 4 - it happens instantly

Regression since 4.3.4f1
There is a workaround - the lag depends on the amount of expanded objects in the hierarchy, but if you put the objects under parents and collapse the hierarchy it reduces the lag