I’m not sure how I feel about this proposal. Previous suggestions regarding zombies and the cold have been extreme, in my opinion. Yours is more reasonable but I still wouldn’t like it.
I think that speed reduction should be capped around a 15% decrease in speed, tops. These zombies aren’t like traditional zombies and I don’t think the cold would have the impact everyone thinks it would.
The current movement speed of enemies is a major component in determining their difficulty. Changing that, even slightly, can be extreme. For instance, when I began playing, I would set zombie speed and durability to 85% in the game creation menu. Even that small change had a MAJOR impact on the pace of the game. I don’t know that I ever really died in that circumstance, because reduced speed affects difficulty so profoundly.