I have two means of importing a Quake BSP asset:

1) Loading it via a button press in the Inspector (EditorBspTest.OnInspectorGUI)
2) Loading it through a ScriptedImporter (QBspImporter)

Both code paths are essentially identical. In fact, loading it through the Inspector actually creates a new ScriptedImporter object and calls the same Load() routine.

Comparing both approaches, the latter is significantly slower.

To reproduce:
1) Load the project
2) Click on "worldspawn" in the hierarchy
3) Click Load in the Inspector. Note how little time it takes.
4) Click Unload in the Inspector to remove everything.
5) Go to the Project view > Assets > Id1 > maps
6) Right click on e1m2.bsp (the same map)
7) Hit Reimport.

Actual: Load time is significantly increased.
Expected: Load time should be identical.

Reproduced: 2018.2.1f1, 2018.3.0b6