Tread pattern/composition, width, pressure IMO
Optimal pressure is very important, but can only be properly set after knowing the first two. If by pressure you mean having it optimized, it is probably #1 or #2. My current tires aren’t fast by any means since they are for more aggressive riding, but the big 2.4 on a 29 does not want to be above 25 PSI for me, especially if the rocks are damp at my local trail and with the EXO sidewalls can handle riding rocky stuff at 22 PSI. For my single speed, since I ride it rigid, I ride at 18 PSI on a 2.3 front to be a little more forgiving and 20 psi rear, also 2.3. I usually try to run a semi slick on the rear when I can get away with it, but have gotten burned not changing it out when conditions on race day were damp and had to walk up a few sections.
When on my 26er, it used to be 25 psi front, 28 rear. One of the biggest advantages for 29er wheels is that bigger contact patch, made even bigger since you can ride at lower pressure.