How to reproduce:
1. Create a new project
2. Import custom package (Import Package -> Custom Package) MyPackage.unitypackage
3. When package is prepared, click on Import

Actual result: Observe that Unity hangs while the import
=>progress bar shows that it hangs when importing basicpack.FBX file and it takes around 5 minutes

Expected result: Importing a FBX file should not cause the editor to hang

Note: you can check Task Manager, it shows that Unity is continuously allocating memory at this point.

Reproducible: 5.3.6p7, 5.4.2p1, 5.5.0b9

Won't fix: Seems that FBX file is >100Mb and contains quite a lot of animations. Most of the import time is spent extracting and converting animations. There is no easy fix for this, but developers will be working on the asset pipeline overhaul soon.