Can't the conjunction ו also mean "but" such that "the boy is slow but the girl is fast" should be accepted? I know this would be true for ancient Hebrew, but I cannot recall if the rule also applies to modern Hebrew, though I seem to recall using the ו conjunction in the same manner at ulpan
I was thinking the same thing. Thanks for asking the question. My Dov Ben dictionary does not give the vav as an option for "but." My guess is that it could mean that in modern Hebrew only when a poet is evoking biblical Hebrew, otherwise the usual way to do this adversative (but) is through one of the options listed in the dictionaries, esp. אבל. I hope a modern Hebrew speaker replies.
I would say not. If you wanted to highlight a relationship between the two facts beyond just placing them next to each other ("this and this", "zeh vezeh") then one way that's quite fine to express it:
"הילד איטי, לעומת זאת הילדה מהירה"
["...le'oommat zot ..."] - something like "in contrast to that," or "while" (however - on the other hand, etc.)
Does this help?
See the discussion below. For your translation, one needs the definite article with both parts of the clause, so that the adjective would be modifying the noun, but we have only one definite article, which means we have a nominal (verbless) sentence--very common in Semitic languages. Also, the copula (הוא) is omitted because there is a tendency to include the copula when there are two nouns, but here we have a noun and an adj.