Please note that immediately after McPherson pulls back, Sherman moves Thomas AoC down this same road through Snake Creek Gap to attack Resaca. Obviously it was possible to move very large numbers of troops over these roads in the same time period.
First, McPherson's men improved the road through the gap after they went through so its capacity got better, but more importantly the volume of men per day that were moved on a given road segment was not larger than the volume of men that McPherson had moved on the first day; there were just more days of movement. Given more days, of course a larger force could cover the same ground.
We know McPherson knew about these issues because he wrote to Sherman about them from the battlefield. What did he do about them before he started?
In April, Sherman wrote to Grant "McPherson has no cavalry, but I have taken one of Thomas's divisions, viz., Garrard's, six thousand strong, which is now at Colombia, mounting, equipping, and preparing. I design this division to operate on McPherson's right, rear, or front, according as the enemy appears."
On May 4th he tells Garrard he will report to McPherson.
On May 5th he tells McPherson of the plan to move through Snake Creek Gap and tells him that Garrard will join him before he moves through it.
May 8 McPherson is approaching west side of gap and expresses concern to Sherman that there is no news from Garrard; Sherman write to Garrard to hurry up
Early on the May 9 McPherson makes his move through the gap and writes Sherman that again he has not heard from Garrard. Sherman wrote back that Garrard would "surely be with you to-day", but he was not.
Late on the 9th McPherson writes Sherman that if he had a division of cavalry he could have done more but Garrard still hasnt shown up and is reportedly resting a days march away, meanwhile Kilpatrick with his cavalry would like to make a try at the railroad and McPherson can move forward again especially if Hooker will hold the gap so he can move more men.
May 10th Sherman wrote him back that "Garrard has moved so slow that I doubt if he has the dash we need in a cavalry officer" and that McPherson should stay put for a day or so.
Are you saying he should have waited for Garrard to show up instead of executing his mission as instructed by Sherman with the resources he has available? How does that show more aggressive action (which was your original criticism, before goalposts were moved)?