I have a very fast scroll runner build for pvp. He can run about 70% as fast as a fully trained mount, and that was before jewelry crafting and the addition of the swift trait for jewelry.
So, I got my main crafter all trained up and just transmuted a couple of the rings on the build to swift, and I don't think he's any faster than he was before the 18% speed bonus the two swift rings give. I'm pretty sure that 18% speed bonus from the rings is wasted on this build. My scroll runner build is Darkstride/Cowards.
So, what is the speed cap, and how do we know if we are hitting it?