In classic porn there always was some kind of plot and pornstars made serious attempts at acting. Heck, they used a mainstream professional actor for a non-sex part in China Girl (probably Annette Haven's first movie). In modern porn, as the OP pointed out, cheesy beginnings are always annoying where the performers are doing worst attempts ever possible at acting. But to me, the most annoying part is blowjob aka fellatio. More than fifty percent of any typical scene is oral sex, in which for the most part a huge cock is staring at you even though you started the movie with the best of heterosexual intentions.