Apologies, I do not know of any sites or critical essays, but I can humbly offer my own opinion. I, personally, do not think that Portia's beliefs on anti-Semitism are at all important to the play. Portia's rival is Antonio, her goal in denying Shylock his bond, is simply to prevent Antonio from winning Banio's affection with self-martyrdom. There is no indication of any prejudice--for or against--his Judaism, on her part. As Jews, in the time the play is set, were confined to a ghetto section of Venice, and she lived in Belmont, it is likely that Jessica and Shylock are the first Jews that she had ever seen.