They have problems everywhere. The issue is when you have guys open, decent protection and your quarterback just stands there like a deer in headlights it doesn't matter what's around him.
We're seeing that asking Fields to run a modern NFL offense is simply not sustainable. He's just sooooo slow at everything that matters. His checks, his audibles, his drops, his processing... There's zero urgency. Its impossible to succeed on offense with those deficiencies.
I guess I look at it like this. I agree with what you state above, but I am in the school of thought that QBs need to have a period of time to learn the game not laying on their back or running for dear life. Rhythm and establishing on field relationships with your receivers is critical. The OC needs to set them up for that success, and I don't believe they have done a good job of it.
If I were the OC with a rookie Justin Fields I would start him off slowly by running the ball of course, and setting him up with short high percentage passes. I would also leverage his mobility and encourage him to use it to throw the ball and buy time. I would work this in the offseason until the routes became mechanical to him. When an Oline and OC are as bad as they have been on the bears any non HOF bound QB is going to be bad. They failed him in his rookie year and beyond that, and I think it's breaking him.
You may be right, his processor may very well be slow, I definitely think Mitch's processor was slow. However, there are so many other problems that I'm not confident he has even been given a reasonable chance for us to make that determination.