Thanks for the info, Lefty. Someone gave me the gist of that before, I guess it must just be coincidence that I've picked up two faster servos for the pair I'm trying to remedy, and the other two pairs are near enough balanced between each other.
For a bit of context: each pair of servos pulls a small block between two points using some thread and reels. The tension only ever gets too high with the servo I'm trying to sort out, either pulling the reel off the servo or snapping the thread.