The two fog spells that cause movement reduction dropped monster movement speed to around the same level as a Stalwart Defender in their tier I defensive stance--it cut monster movement by half or more.
At some point not very long ago, the spells stopped slowing monsters down to this degree. Now, the movement reduction is hardly perceptible; I'm guessing the reduction is not more like 5-10%, making this portion of the spells' effect almost entirely useless.
As a specific example, it used to be possible to kite stuff back and forth through a fog, such that the monsters were slowed enough that you could get them halfway through the fog, run around them, and repeat when they got near the edge. Now, stuff gets through the fog so quickly, you might as well be kiting without it.