Major events do happen quite fast in this act.  In order for Macbeth to take over the throne quickly (to fulfill the prophesy), he must be rushed to Scone to be invested.  There are all sorts of little events that happen along the way if the reader isn't paying attention.

Banquo and Fleance feel from the first scene that things are not right.  Then Macbeth kills Duncan.  But as he goes to his wife to tell her it's finished, he is still carrying the bloody daggers.  They have to wash up, change and appear as if they'd been sleeping when someone comes knocking early in the morning.  Then Lennox tells Macbeth, after Duncan's body is found, of the strange occurrences in nature. 

After they find Duncan's body, Macbeth also murders the two guards that were "supposed to have" killed Duncan.  This isn't explained much, and neither is the whole trip to Scone.  All of this is compressed so that Macbeth can take over as king and "get away with it" before anyone puts two and two together.