Thanks -- I hadn't thought of it in terms of speed before. That said, I still think allowing horses in popular wilderness is a bigger mistake than allowing bikes. They turn a trail into either a trench full of shit-powder, a well-churned bog, or a set of awkwardly high and long steps meant for horses rather than humans.

It can definitely be an issue and a point of conflict in popular areas. It may be an issue that needs to be resolved with permits—limiting to a certain number of horse users per day per trailhead, for example. I hate permits, but if visitors are beginning to significantly impact the Wilderness (which the Wilderness Act is established to protect against) then permits to reduce impact are necessary.

Where I live, most Wilderness Areas are relatively little used and suffering from post-fire erosion, and so the trails often suffer from overgrowth and fading tread. As a result, horse use is actually quite welcome as it's currently below the threshold where it goes from helping keep the trail beaten in to actively damaging it.

You're lucky to live near relatively untrodden wilderness. In my experience as a hiker, many trails in the Sierra Nevada have been beaten into dusty trenches by horses, mostly thanks to pack companies. Quite a few in the southern Rockies have clay-rich soil, which horses will destroy when it is wet (so will bikes, but mud-riding is awful, so most riders will stay away). I have also been in some areas where horse traffic is the only reason that unmaintained Forest Service trails have not grown over.

I think banning commercial packers from the Wilderness would go a long way to fixing the problems with horses. Perhaps with some sort of licensing system, mountain bikers could be allowed as well.