Maybe there ought to be another level for folks who have won awards from recognized societies and associations such as Nobel, ACM, what not
I often run into people who ask me about my immigration status and when I say that I am waiting for my GC to be done, they nod sympathetically, roll their eyes and tell about someone they know who "had to wait two years! Gosh!!". I tell them, with as much politeness I can muster, that I have been in the country for over 10 years, have filed for my GC 5 years ago,, and expect to wait another 5 to get it.
If the US didn't have a considerable amount of immigrants from those countries then the times would be 8x less.
Same happens with Mexico. There are so many Mexican immigrants in the US already (legal and illegal) that getting a GC it's almost impossible now unless you have a legitimate marriage to a citizen.
No, past levels of immigration has no bearing on the backlogs. For example Germany, Ireland, Italy etc have no skilled immigration backlogs.