Designers should be very familiar with the standard widgets, idioms, and interactions of the platforms they're designing for, but I don't think performance is rightfully something they should have to worry about.
>>always+(OP)
> but I don't think performance is rightfully something they should have to worry about
But then I, as a developer, have to argue with the designer why a table containing a thousand rows or more, with no pagination, may not be a good idea :-(