I think it would help out with a huge amount of the frustration that some people are experiencing in Dominion. It's not that it is impossible to win against super speedy champs, it's just annoying
I just think it's too early to make this kind of observation. In my personal games, I only see speedy champs being a problem when teammates constantly overextend, leaving us open for back caps.
If both teams over extended and constantly trying to solo-ninja 5-cap, then yes, it'll get annoying that the team with the faster champs seems to be winning. However, I think that's more of an issue with a teams strategy or understanding of the metagame than a broken game mechanic.
I think we have to be careful what we mean by "abuse". Some champs have extra speed for a reason. To make up for a lack of other disadvantages. In a game like this, where mobility is viable, if you nerf the speed issue to much, there will be no point in taking Speedy champs or stacking speed items on speedy champs. Which could then mean back dooring in general won't be viable at all. It'll be 5v5 Tanky DPS sitting on 2 points, constantly battling for Windmill.
What is the exact point we can nerf this Speedy gameplay, without making it pointless and just removing that tactic completely from Dominion? Or is this exactly what you want? To remove the super fast backing capping tactic.