They don't speed up or slow down, the speed just seems different because the screen is scrolling.
If you want them to move at constant speed relative to the screen, you can put them on a layer with zero parallax.
By the way, if you have Platform behavior on snakes, you should use Platform actions to move them, like "Simulate control". "Move forward" and "Move at angle" are not Platform actions and may not work very well.
Also, when moving something on every tick you should use delta time. Currently your snakes will move at different speeds on monitors with different refresh rates. If you run the game on a 240hz monitor, they will move 4x times faster.