Economists have known for a long time that there is a cyclical asymmetry in the unemployment rate. In a recession, the unemployment rate tends to spike up quickly and sharply. During an economic expansion, the unemployment rate tends to decline only gradually. Consider the following data, for example:
The shaded regions roughly depict the periods over which the unemployment fell from peak to trough. As you can see, what the U.S. is experiencing right now looks a lot like what Canada experienced in the early 1990s. Evidently, it takes time to rebuild the employment stock after a shock. And the bigger the shock, the longer it seems to take.