I was told that using single foot guides on a fly rod would slow the action vs using snakes.

I would have guessed the opposite. (The snake guides are longer so I would think they would add a little stiffness, that this would affect the tip relatively more and move the load point further down.)

What am I missing?

Thanks in advance for your input.

I seriously doubt that (m)any casters would notice a difference between snakes and single foot guides either wire or ceramics.

No, adding guides will not change the action, but will change the speed. Guides never add stiffness. Impossible. They always slow the speed (reaction and recovery time) so you want to add as little additional weight as possible. Whatever set of guides adds the most weight, will reduce the speed by the greater amount.

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Also single foot guides will only have one thread wrap with finish Snakes will have two wraps Cuts the weaght in half Weather it can be noticed or not

Bill - willierods.com