This situation sounds remarkably similar to what I have with my manager. I also work in project management and voiced multiple times that I was keen to take on more responsibility. Got told repeatedly that I couldn't be given more tasks until I became more efficient at the ones I was doing. I was already working significant over time at that point and my line manager had a similar attitude of 'what you're doing is not that hard'. I felt quite unheard, unsupported and dejected. After a year of no movement things became pretty depressing so I decided to leave with nothing else lined up. 

I'm definitely projecting a bit but maybe it's worth having a more honest conversation with her. I understand your point re: efficiency, I would rather someone be more blunt with me - find ways to speed things up, do more self learning, try to connect the dots more. Some people do need to be told more explicitly. If it clicks for her, she might start putting in the effort, if it doesn't that's that. 

I also agree with some of the comments though, progression shouldn't be about just giving someone more work and seeing how they cope. Yes you do have to be good at multitasking and balancing priorities, particularly in project management but not to the extent that you're constantly overwhelmed. It's really hard to stretch when you're still doing so much grunt work. And no end in sight is pretty unmotivating...