It may not seem like there are that many cars merging, but it adds up.
I suppose it does.
Combine that with seemingly most drivers lack of understanding re: the merge process* and that will lead to slowdowns.
*it seems most drivers instead of adjusting their speed, would rather pull into a different lane when faced with a merging vehicle. In my driving experience, this seems to be happening EVERYWHERE, not just Chicago. Somehow drivers have lost their ability to gauge the speed of a merging vehichle.