I can't imagine Alice fighting unless she absolutely has to, if only because a fight between the two is surely to cause massive damage and to put everyone nearby at significant risk.
Alice has already made it clear that she doesn't think that she and Sedna, TOGETHER, are a match for He Who Looms Behind. Whatever rescue methods she employs, they probably won't be a direct fight if she has any choice in the matter.
As to Loomis' speed, I have to wonder if the reason he just appears behind you is that he moves silently in an eyeblink to that position. Supporting evidence:
http://www.alicegrove.com/page/8 shows Loomis appearing out of nowhere behind Alice, and she susses that he's stronger than her, right away.
Maybe she knows him, or recognizes the model. But maybe not. Maybe instead she is just really observant (soldier training: you watch your six), and KNOWS he was nowhere nearby a quarter-second ago.