I agree with you two! JA really made it to make Thorpe suspect to the reader from the very first sentences he speaks.

I also noticed that its always the same about narcisstistic men - today they are talking about their car, back then it were their gigs and horses ;)

And I think Catherine is not yet able to make out a person`s character by the very first sentences she speaks, therefore it takes her a little longer to find out. And later she is aware of not liking him at all but keeps silent in respect for her brother and, I think, her insecurity regarding to evaluation of other people.