When my head gets all stuffed up from allergies, I liken the experience to moving through jell-o. Sadly, as you might guess, I have never actually walked through jell-o so I couldn't say if this is truly the experience I think I'm experiencing. For all I know, walking through jell-o would leave me dead in several minutes from suffocation.
I started using this expression when I was visiting cousins up in the UP. (Yes, I was sick constantly up there for several years. Now it's better because I've built up a super immunity to those northern bugs). I had a rather high fever and my head was really stuffy. Every effort seemed to take a little bit longer to accomplish.
I would think, "Let me walk over to that chair and pick up a comic book," and then my brain would rely that message to the rest of my body at a speed that can be compared to rush hour traffic. Slowly my limbs would understand my intentions and start creaking forward in a movement somewhat comparable to walking.
My body just felt so heavy and useless that by the time I had moved forward the six steps to the comic book, I felt dizzy and confused. I sat down next to the chair since the effort to raise myself slight to sit on the chair seemed to be an insurmountable ordeal and flipped the pages of the comic.
My mom, who was taking good care of me, came back with some water and ibuprofen for the fever and asked me why I had sat on the floor. I looked at her and told her that I felt like I was walking through jell-o. She laughed and helped me to the couch to lie down for a bit.
As with the majority of cold and flu bugs I've had over the years, I felt much better after some rest and once I got up, it was as if someone had thinned out the jell-o barrier around me.
I think they replaced it with cotton candy.