We know that Mr. Rushworth is not the brightest bulb in the room, but is it all ignorance and self-delusion? Even before Henry comes into the picture in a big way, Maria's behavior to him is clearly neither loving nor conciliating. Miss Bertram’s attention and opinion was evidently [Mr. Rushworth's] chief aim; and though her deportment showed rather conscious superiority than any solicitude to oblige him, the mention of Sotherton Court, and the ideas attached to it... prevented her from being very ungracious. Chap. 6

Afterwards, his fits of jealously clearly point to the fact that he notices the flirtation between Henry and Maria. But, after Maria passes into reckless behavior, his lack of any kind of action seems more than a little surprising.

From this moment there was a return of his former jealousy, which Maria, from increasing hopes of Crawford, was at little pains to remove Chap. 18. Her behaviour to Mr. Rushworth was careless and cold. Chap. 21

Recalling NA again, Isabella's flirtation with Cap. Tilney leads Isabella and her fiancé to part with mutual consent. After all, many people think Fanny ought be more proactive about her abusive situation in spite of her timid nature and constricting circumstances. Does a little "slowness" excuse Mr. Rushworth from taking any kind of direct action?