This Illinois % speed issue is because of slow DSL speeds from AT&T, Frontier, and other ILECs in non-Chicago areas of Illinois.
Since you live in TN, you probably don't know that your Chicago "80% of Illinois" population figure is not right. We non-Chicago
area Illinoisans have a long-standing anti-Chicago area attitude, and resent that non-Illinoisans believe that Chicago
takes up most all of the Illinois population.
Per Wikipedia research today, latest Illinois population breakdown:
60% of Illinois -- 8 Million in Chicago-Naperville-Joliet (all Illinois) MSA
40% of Illinois -- 5 million in Illinois(outside of Chicago and collar counties)
13 million total population for State of IL
Downstate Illinois Major Metros, ILEC in that city, and the ILEC for its suburb/rural areas:
Rockford(AT&T), Frontier in most of surrounding area
Springfield(AT&T) , Frontier in most of suburbs/rural
Peoria(AT&T) Frontier in some suburbs/rural
Champaign-Urbana(AT&T) Frontier in most of area
Decatur(AT&T) Frontier in some suburbs/rural
Bloomington-Normal(Frontier) Frontier in most of surrounding area
DeKalb(Frontier) Frontier in most of surrounding area
Quad Cities-Illinois side(AT&T), Frontier in some suburbs/rural
Belleville(AT&T) , Frontier in some of area
Note: AT&T has U-Verse in some relatively small areas of Springfield, C-U, and Decatur. Chicago suburbs have much more
U-Verse availability. Verizon never made FiOS available in Illinois, because of inability to effectively lobby the Illinois Legislature.
AT&T has major sway there. Verizon(and GTE) were both less effective in Springfield, than AT&T.