It's really only a difference of like 3 frames, 2 is still pretty fast. The advantage is the range it has. When you have a string that's pretty fast, has better range than the best normals of all the rushdown characters, and is hit-confirmable into a 45% (give or take some %'s depending on preferences) combo, she becomes one of the best close-up characters in the game.
And I totally agree, but those are points other than the actual speed of the attack that make it good.
@Pig:
I said Raiden's strings were interruptable.
I agree with you on Mileena. I play her and think her pressure is great, but several others disagreed telling me that I was only getting by with my pressure strings because it was online. Since you agree, I will change this.
As far as high learning curve. I understand it isn;t really a weakness, but it was requested by others that I tag the harder to learn characters. SRK lists something similar as a weakness for C.Viper.
Also, Mileena has to have something listed. She has slow normals. This is something that someone looking to pick her up should be aware of, imo. I do realize that she would be amazing if she had an armored special, but it is something she lacks, and IMO, worth mentioning.