/cheer! wonderful to hear
+1 I am embarrassed to say this took me several days to find. Days which I could have been trying to play, or have at least seen if disbanding the party fixed the overburdened to 1.0speed bug. Days I spent cursing the "special" programmer whom I had thought removed the button. Days which made me feel like a real dumbass when I finally found where the damn thing was hidden.
I started another run from scratch in 1.8.0, and noticed 1.0 speed groups in under 70 days. (Wasn't really looking for them, but since I was aware of the problem, I saw it much sooner.) I think it starts snowballing immediately day 1, but players don't notice until we are ready to actually interact with the different kingdoms as mercs or vassals, then we see the overloaded parties once we have built up to where we might interact with them.
Opened a ticket and sent along a save where I believe my companion party, and several other of the Sturgian parties were affected and stuck at 1ish speed. Only newly reformed parties are traveling at normal speeds, many Vlandian and Sturgian parties were stuck at 1.0 and overburdened. I am only around day 700, so they are not hoarding millions of grain, only hundreds to thousands. The only way I could get my companion party back to a normal speed was to fully disband, and reform it. (HORRIBLE location for the disband button btw, completely non intuitive to look there, thought for days it had been removed) After reforming, they were fine until a fight with an affected party, then it was back to being over burdened and slowing down the whole army.