Exactly TB. Also, as the novel states (yes, the novel) "It was almost three years since he had returned home ... The three years had been spent in learning the Family business."

One should also consider that waiting almost a decade to exact revenge might invite the following:
Michael and/or Vito's murder or death otherwise (his, it did)
Michael and/or Vito's incarceration
The unrecoverable denouement of the Corleone family ("The Corleone Family was definitely in decline.").
Treachery (which waiting just a few years did invite)