The mushrooms are quite different from the slime molds, even if the humans, the mushrooms and the slime molds are much more closely related between themselves than any of them is related e.g. to the plants.
Like most animals, the slime molds have retained the primitive way of life of the eukaryotes, i.e. they have mobile cells that can hunt, catch and ingest other living beings.
The mushrooms, like the plants and a few other groups of eukaryotes, have reverted to a way of life similar with that of the bacteria, i.e. they have developed a rigid cellular wall, which protects their cells, but which prevents the movement of the cells, so they can no longer ingest anything else but small molecules, so in order to eat they must secrete into their environment enzymes that will digest their food outside their body, resulting in small molecules that can be absorbed.
Like most plants, most mushrooms cannot move, they achieve the equivalent of very slow movements by directed growth. The exceptions are the few carnivorous plants and the few carnivorous mushrooms, which have traps for their victims having a relatively fast action, caused by various kinds of springs.
The slime molds move around and catch and eat their prey, behaving like giant amoebae.
The eukaryotes with mobile cells, without cellular walls, have visible reactions to whatever they sense in their environment, e.g. by pursuing what they want or running away from whatever may be dangerous.
Those like the plants and the mushrooms are also likely to have various sensors, but any reactions are too slow to be perceptible at the human scale of time.
So the slime molds and various mobile unicellular eukaryotes, like the ciliates (some big ciliates are as big as some very small multicellular animals, like the rotifers, and there is little difference in behavior between them), can be considered as higher in a scale of sentience than the plants and the mushrooms.