Three relevant pieces of information to add to this discussion.
First, Maj. Gen. Trimble. Lee met him mid-morning on July 3 and the two of them went to the cupola of the Alm's House for observations before returning to Ewell's headquarters, who had been absent. Just guesswork, but that was valuable one-on-one time for these men. Within an hour or two, Trimble received orders to take command of Pender's division and did so in the late forenoon. It was probably then that he visited his new command: "Trimble rode down the line and halted at different regiments and made us little speeches, saying that he was a stranger to us and had been sent to command us in the absence of our wounded general, and would lead us upon Cemetery Hill at 3 o'clock."
Next, I recently came upon this item, by W. H. Jenkins, a courier assigned to A. P. Hill: "The morning hours had been spent ... in massing the troops and arranging for a decisive effort about the noon hour or a little after. ... At A. P. Hill's headquarters a courier was called, and your correspondent answered, and the General said, 'Here is a circular message to the commanders of the artillery which you will deliver.' He handed me an open order to the officers of the Third Corps artillery, which I at once proceeded to deliver. This order was short, directing the artillery officers to open fire upon the Union lines at a signal which was to be given at a certain time from the Washington Artillery." At a minimum, the courier would likely have visited Col. R. Lindsay Walker and the five artillery battalion commanders spread out along the line for some 1.5 miles. This took time.
Finally, there is the question of Culp's Hill. Johnson, bolstered by Walker, O'Neal, Daniel and Smith, fought hard until nearly 11 a.m. - Lee may have been waiting to learn the outcome of this effort, which if successful, would have greatly altered the tactical situation for the Confederates. At the very least, it tied up and wore down significant Federal forces that otherwise would have been fresh and readily available to counterattack the charge of Pickett/Pettigrew/Trimble, in other words, improving the latter's chances of success.