The only way you would know for a fact as well as you say you do is if you got stabbed like that. So calm down. In reality, since the stab just happened, the blood would not have had enough time to get into the mouth like that. If you notice, the blood is oozing out of her mouth. Blood from a wound like that would take a few seconds to get to the mouth, even a minute at times. This is mostly because that a blow like this sometimes causes the victim to cough.

So, lets take a look at two things in this pic: 1: The blood from her mouth is oozing out (drooling if you will for those that can't type out words or use capital letters or correct punctuation), this suggests that your lungs, windpipe, or esophagus have been damaged badly enough to force you to be coughing for a while, 2: The blood from her wound in this picture is flying out in an explosive manner, suggesting that the thrust just accorded in time with this picture.

With this information, we can conclude several things. First, she was most likely already stabbed in the back before the final thrust, the first stab damaged either her lungs, windpipe, or esophagus, and that before dealing the final blow, her killer toyed with her for a bit of time, enough time to allow her to cough up enough blood to pool in her mouth to get the oozing/drool effect we see here. Two, the artist most likely did not know or research this indepth and added the blood from the mouth to make it look "cooler". Three, she most likely was only stabbed once due to her surprised look. Four, three contradicts my first statement.

So, in conclusion, the artist did the blood from mouth effect wrong. Speed has nothing to do with the stab, the force behind it does the work, a fast but weak attack would not be able to do the damage required to force the weapon all the way through, nor would the speed cause the blood to fly around like that, nor would it cause blood to rush into the mouth at the instance of the attack.

So, one of two things is the truth; One: She was stabbed twice, and we are seeing the final blow, but she never got over the shock of the first strike, thus her facial expression, or Two: She was stabbed once and the blood from the mouth was the artist's attempt of making the picture look more bloody/cool.