In this scene, Lady Macbeth is sleepwalking, and she seems to be, in part, reliving the night that Macbeth murdered King Duncan at her behest. On that night, the couple planned to commit the murder in the wee hours so that Duncan would be fast asleep and his grooms would be thoroughly passed out from the wine and wassail with which Lady Macbeth plied them. Just before Macbeth did the deed, he said,

The bell invites me.
Hear it not, Duncan, for it is a knell
That summons thee to heaven or to hell (2.1.75-77).

It seems that Macbeth may have heard the tolling of the clock, as his wife would have as well. Therefore, in her dreamy sleepwalking state, she seems to hear the clock chiming again, and so this is why she says, "'tis time to do it." Then, in her dream, she jumps a bit forward in time, recalling her words to Macbeth after the murder, when she says that they need to hurry up and get to bed because they would soon be called upon (and they must look like they've been asleep). At the time, she said to Macbeth,

I hear a knocking
At the south entry. Retire we to our chamber . . .
Hark, more knocking.
Get on your nightgown, lest occasion call us
And show us to be watchers (2.3.84-91).

She did not want to be caught awake, especially after she heard someone knocking. Now, in her dreams, she hears the knocking again and tells Macbeth to get into bed so that no one sees them up and about. She is reliving highlights from the night of Duncan's murder because this is one of the events that weighs so heavily on her conscience.

Lady Macbeth's guilt is coming out in her sleep. These lines show the disorder in her mind. Interspersed between her refences to Duncan are references to her other deeds. In her dreams, even the original deeds have changed. Where once a little water washed her hands clean, now all the perfumes in Arabia won't work. The knocking at the gate she refers to has more than one meaning. It is a direct reference to the porter in Act 2, and yet this time it is different. She needs to go to bed, to put the problem to rest (Even in her stressed state, she counsels this, telling herself that what's done cannot be undone). Yet just as Macbeth couldn't follow her advice earlier, she can't follow her own now. Earlier in the play, the knocking at the gate was a reminder to hide so that she wouldn't appear guilty; now there is nowhere to hide. She can go to bed, but she can't rest.
The gate has become the gate to her subconscious: something wants out.

This is Lady Macbeth speaking while sleepwalking. She's recounting the events the night they killed Duncan. Remember that they felt rushed to kill Duncan without anybody learning of the deed. Once he was dead, they heard someone knocking at the gate and had to rush off to bed to pretend they'd been asleep the whole time.