Hi,
is it because you are editing a prefab and this prefab has many instances in the scene? that's a known cause for playmaker editor slow perfs, if that's the case, upgrade to 2018.3 or more the prefab editing system is now separate, or else load an empty scene, edit your prefab and go back to your scene, I know it's a pain, but I am not sure there are any alternatives right now.
Bye,
Jean