Here's some further evidence of Lanfear's strength in the Power...

When Moghedien Travel's to Graendal's lair she weaves something similar to a doorbell to ask permission to arrive. Graendal is surprised and sends a reply allowing her to weave her gateway. Moghedien enters and looks suspicious.

When Lanfear Travels to meet Rahvin she merely pops up behind him, and half scares him to death. He fills himself with saidin, and prepares to destroy the intruder, and is angry that she surprised him.

So it seems that Lanfear isn't scared to surprise a male Forsaken like Rahvin. He's prepared to kill her, but she just stands there daring him to try. This is probably a hint at her power compared to someone like Rahvin - she goes where she pleases and when she pleases. Moghedien is scared that Graendal will get a surprise and turn on her, blow her up, and then see too late that it was Moghedien.

Other explanations for Lanfear's behaviour...

1. She's just plain rude.
2. She's mad.
3. She overestimates her abilities - and she's just lucky Rahvin didn't want to dance the spears.