Stupid, slow, dull, dense, crass, dumb are comparable when they mean conspicuously lacking in intelligence or power to absorb ideas or impressions or exhibiting such a lack.
Stupid can apply to a sluggish slow-witted lack of intelligence, typically congenital or habitual, or it can apply to a usually more or less transitory benumbed or dazed state of mind that is typically the result of drunkenness, shock, or illness; although the term seldom is applied to the insane or the imbecile, it often also suggests senselessness.
Slow implies stupidity manifested especially in lack of quickness in comprehension or reaction; often the term is little more than a euphemistic substitute for stupid .
Dull (see also Dull vs Blunt vs Obtuse and Dull vs Humdrum vs Dreary vs Monotonous vs Pedestrian vs Stodgy ) suggests a labored slowness or sluggishness of mind that may be constitutional, or the result of lack of mental exercise, or of overwork, or of a physical condition, and is unrelieved by any hint of quickness, brightness, or liveliness.
Dense implies a quality of mind that makes it impervious to ideas. Additionally it may imply qualities (as obtuseness or stolidity) that reveal lack of perception, sensitiveness, or subtlety.
Crass suggests a gross unfeeling quality that makes the mind incapable of delicate mental processes (as analysis, discrimination, and evaluation) or impervious to refined or spiritual ideas.
Dumb (see also DUMB 1 ) is a term of contempt that may be used in place of any of the preceding terms especially when obtuseness and inarticulateness are also implied.