I absolutely would tell her.
Tell her the first thing the next time you see her.
"There's something I wanted to talk to you about...."
Take the plunge.
It's really important to ask for what you need. You need her to slow down.
I don't know why you think this would be "criticizing" her. It's just information for her that will help her help you. You're not telling her, "You talk too fast therefore you are a horrible person."
You're just saying, "Hey, for me, these ideas are new and it would really help if you could slow things down when you speak with me so I can take them all in"
I think you should tell her that what she perceives as your "flat" affect is just you feeling overwhelmed at how much information she is giving you, so quickly.
I also think you should ask yourself why you feel so hesitant about bringing this up. Do you think she will take offense or reject you? You might want to talk about that, too. Personally, I would want to know if I was talking so fast that another person could take in what I am saying. There are all sort of personal differences. In the US, "northerners" tend to talk faster than "southerners." There's no right or wrong to this.