I improved more rapidly
This is a nice detail in itself but not wholly devoid of further import. However boastful the Creature may be about his advancement, the important point, and one that has been slowly dawning on the reader, is that he is extremely intelligent. Like his "father," he is superior to his fellow-students (I:3:2). With his great stature, his exquisitely fine feelings, and his mental quickness, he should have been equipped, in Victor's early aspiration, to be a "new species" (I:3:8) of superhuman being. Instead, he has been spurned.