Here's a simple specific example. Take the Haskell expression [1..n], for some big n. There is no general answer to the question "what would be the performance implications of replacing [] with [1..n]" – it depends on the context of [].
In a strict language, you can say (roughly at least) that replacing [] with [1..n] will add at minimum a certain number of extra CPU cycles and a certain amount of additional allocation. This kind of reasoning is pretty rough and ready, but it is accurate enough to be useful for reasoning about performance in practice.
I note that Simon Peyton-Jones has said these exact words in a public talk:
"Laziness makes it much, much harder to reason about performance."
I think it's extremely unlikely that he's mistaken or confused on this point.