Should something like this be considered, which I find to be an intriguing idea, I would suggest that infiltrators have the power to reduce or increase travel times IN the region they are in. Whether leaving or entering.
AKA. They and the army are in region 1, moving in region 2. The infiltrator could changing travel times in and out of region 1.
Or they could move to region 2 and change travel times going into or out of region 2 once there.
They can increase travel times, though they have to been travelling between two regions, IE they slow down movement on the road they are on. This was my key responsibility as a army infiltrator when I was in Arcaea. Many a time I ensured the enemies largest units did not arrive in time for battle. I never actually stabbed anyone in all my time as a infil.
