Actually It was 27 years of recovery, the true form needed to recover from being mentally assaulted by the psych Power of a 1-A being above him in every single aspect + The Ritual of Chüd.
Also, Pennywise can create Avatars instantly heck he can even create multiple Avatars, as he can cause multiple beings to perceive it as multiple different shapes at once:
Richie chanced a glance behind him as he flung himself onto the package carrier and saw the Werewolf crossing the lawn toward them, less than twenty feet away now. Blood and slobber mixed on its high-school jacket. White bone gleamed through its pelt about the right temple. There were white smudges of sneezing powder on the sides of its nose. And Richie saw two other things which seemed to complete the horror. There was no zipper on the thing's jacket; instead there were big fluffy orange buttons, like pompoms.
~ IT
So once again Bill Denbrough found himself racing to beat the devil, only now the devil was a hideously grinning clown whose face sweated white greasepaint, whose mouth curved up in a leering red vampire smile, whose eyes were bright silver coins. A clown who was, for some lunatic reason, wearing a Derry High School jacket over its silvery suit with the orange ruff and the orange pompom buttons.
~ IT
These two characters were telepathically linked, so each of them started seeing bits of what the other was perceiving.
And also, he could do multiple actions at once. Both manifestations were performing actions separately.