I believe J doesn't have any optimizations specifically for the problem of large temporary arrays (not to say that such can't exist, but I don't think anyone's

I believe J doesn't have any optimizations specifically for the problem of large temporary arrays (not to say that such can't exist, but I don't think anyone's written such yet). With multiple temporary operations, each making a temporary array larger than cache, it could in fact end up slower than mediocre imperative scalar code.