There won't be a discernable performance difference, when using a decent compiler.

    for (int i=0; i < data.size(); ++i)
        data[i].Foo();

Not slower, just profile that! Simple code tends to be quicker. In this case, all versions are equally simple

    for (Object& it : data)
        it.Foo();

Fixed that for you: use a reference to avoid unwanted copying. This is roughly equivalent to:

    for (auto it=std::begin(data); it!=std::end(data); ++it)
        it->Foo();