I think Frieza could've destroyed Namek instantly if he wanted. That was a deliberate slow destruction on his part. Vegeta is plenty strong enough at this point in the story to essentially instantly destroy Earth.

We see Vegeta and Nappa standing in their open pods out in space when they destroy that bug planet on their way to Earth, so presumably they can tolerate being out in space at least for a little while too, so maybe his "plan" was too call up his pod to his position after things blew up.