>>386335312
^this anon is correct
to be more specific, the face and all its shapekeys should be a separate mesh from the rest of the body. the rest of the body should have zero shapekeys. this adds an extra mesh total but is better on CPU performance because there are less vertices affected by shapekeys.
if you have 1 mesh, unity will have to go through all the vertices to apply shapekeys, even parts of the mesh that have no shapekeys affect them (such as the legs and torso).
additionally, if you have any shapekeys always active, consider setting them as the new basis with CATS so that they aren't shapekeys at all anymore.

if you really want to minmax this, combine shapekeys in blender using "New shape from mix" to have 1 shapekey per facial expression (rather than separately having eyebrows, eyes, mouth etc all on different shapekeys for a happy face).