yes and no.
the reduced speed is a possibility, but not necessarily something that would be there the sole purpose of a trench is to defend you from those shooting, sandbags are a obstacle that can be easily destroyed while trenches would take some time to be destroyed (more so with explosives than with small arms fire) and actually it wouldn't really incur the same bonus because sandbags are out in the open and can be shot at while trenches aren't they are dug into the ground which makes you a smaller target to hit.
the reduced speed is a possibility, but not necessarily something that would be there the sole purpose of a trench is to defend you from those shooting, sandbags are a obstacle that can be easily destroyed while trenches would take some time to be destroyed (more so with explosives than with small arms fire) and actually it wouldn't really incur the same bonus because sandbags are out in the open and can be shot at while trenches aren't they are dug into the ground which makes you a smaller target to hit.