Absolutely the speed of the torpedo is important. It isn't much good if it has a range of 50km at 25 knots if its target has a top speed of 30 knots - lol.
The issue is, will the launch platform get close enough to its target and survive long enough in a hostile ASW environment, to be able to fire one of these weapons. Iran's conventional submarines are not bad, but they are limited in their range and payload and will also need to operate in the littoral zone so as not to be simply annihilated by US nuclear subs which are at their best in the deeper waters.
Torpedoes, just like surface-to-air and surface-to-surface weapons, are classified as smart wepons. But there is always a compromise in designing and building one of these weapons. If you want to keep it's size small and its speed high so that it is harder to intercept, then your range is going to be dramatically reduced. The faster and smarter you want it to be, the larger it is and therefore the easier it is to detect and destroy before it reaches its target.
And there is no such thing as a missile that can't be intercepted. Some are just harder than others.
Personally, I think the articles on both the torpedoes and the surface missiles are poor attempts at propoganda, carried out by people without the faintest understanding of modern naval warfare.
