The print sheets won’t work unless you have the Use Smart Object checked. Without it, it will only put one image in the template and not duplicate them to the duplicate images. The duplicates are created by the duplicated smart objects. So turning it off won’t work for this template.
Was the template on the network drive too? Try putting the Print sheet template the desktop and the select that in SPA. Then also save to the desktop. Let me know how fats that runs.
My test ran in 3.06 second per image on the same print sheet with Use Smart Object On. There is no reason that option should make it take 40 seconds. If using that option causes this much of a slowdown on your computer then something is very wrong.