If you are going to frequently modify an On-The-Fly object (i.e. Change the vertex position within a timer) then this command will help speed up rendering.
#Create a triangle
viz.startLayer(viz.TRIANGLES)
viz.vertex(-1,0,0)
viz.vertex(1,0,0)
viz.vertex(0,1,0)
triangle = viz.endLayer()
#The contents will be changing frequently
triangle.dynamic()
def changeVertex():
#Change the position of a vertex
triangle.setVertex(0,x,y,z)
vizact.ontimer(0.1, changeVertex)