As far as I've seen, ants use absolute distance, not relative distance to see how to get somewhere fastest. All tiles are the same size in distance, but not in travel time. So ants try to walk the shortest path rather than the fastest. By expressing tile size in speed on that tile instead of actual tile size the ants will likely walk the fastest path.
So if the player were unable to place fast movement chambers on the shortest path, the ants will still use the fast move chamber as long as it is faster than the alternative.
So if the player were unable to place fast movement chambers on the shortest path, the ants will still use the fast move chamber as long as it is faster than the alternative.