Higher than "simple to medium" scale/complexity situations are probably their priority when it comes to optimisation, on the basis that most smaller scale/complexity apps won't experience the difference and those that do can be tweaked by the programmer. A major factor to consider when optimising anything particularly something as generic as a large framework is that an optimisation for small scale situations can have a detrimental effect on larger ones (and vice-versa, though as problems are more noticeable on larger scales of size or complexity optimising for that at the expense of the smaller situations is the better over-all choice in many cases).