The other day I did a flight from KSTL to KORD using FF757 and once I got about 10 miles from KORD my FPS dropped form a rocksteady 30fps to 4fps. I thought well it must be another bad Airport I downloaded as I had one Airport who for some reason adds 1.5GB to my VRAM usage. So I deleted my KORD airport and placed my plane on the runway and still only could get about 5 fps. Long story short I added the new HD Mesh V2 for the region, but I still had Simheaven's OSM-North America in my custom scenery and for some reason they would'nt play nice together for that one particular area. Removed OSM-North America and fps went back to 30.

I have had a few problems in the past with OSM scenery near Switzerland and France dense areas, before starting to use SimHeaven's OSM scenery sets, but I never had problems with the SimHeaven sceneries, OSM or photo.

 

I do notice though an impact of the Mesh v2 in my system ( GTX 650 Ti 1GB "only" ), and that's why I reverted back to default mesh.

 

Also tried SimHeavens new mesh v2 for photos, but again, I had to give up because of the lower fps.

 

Scenery for me is not at all the most impoirtant factor, other than the correct depiction of airports and main orography, city landscapes, country landscapes... :-)

OpenstreetMaps for Chicago had a huge import of every building in the city from some government source early this year, so what probably happened as you approached Chicago is that all those buildings started to be drawn and goodbye framerates. 

 

It's a similar situation in Paris or London. I find the draw distance on the OSM scenery from Simheaven.com too much, which is why I generated my own using a much smaller LOD of 11km.