Run/walk through treacle
"Treacle" is a thick, sticky dark syrup made from partly refined sugar; molasses. It is something that would be very difficult to walk through and is therefore a metaphor for something that slows progress.
"Last week everything was easy and we were making good progress, but now it is like walking through treacle"
"Trying to organise my new visa is like trying to run through treacle. It is taking so long!"