Often (but definitely not always), requests to peer-review are accompanied by statements like "We consider our Journal X to be on par with Journals Y and Z, and aim to be extremely selective. Please only recommend publication if blah-blah-blah...."

I'll then read the introduction. If I don't have sufficient expertise to referee the paper, I'll decline the request. If I do, then I'll see what the authors have accomplished. If, in my judgment, this doesn't rise to the level that the editors asked for, then it doesn't take me a long time to decide this and say so.

Conversely, if the introduction does impress me, then I will want to check the proofs in very close detail. In this case I will commit to writing a detailed report in the future.

It's quite common for papers to be declined from individual journals; it's happened to me plenty. There are tons of other journals out there; you can submit somewhere else. And when I decide that a negative report is called for, I write it right away, so as to not keep the authors waiting forever.