Updating Smart Objects: Sometimes a Second, Sometimes 5-7 Business Days

I have a pretty simple setup for making product images for a client I'm working for. They sell products in boxes, and I have mockups of those boxes from several angles. So a typical PSD file is an image of a box where the 1 or 2 visible sides are smart objects transformed to fit the perspective, which are just another 'master' smart object (the diecut design, also a standalone PSD file) cropped. Super simple.

So when I replace the diecut with a new design and save that PSD file, the smart objects that link to it update. Sometimes this takes just a second or two. Sometimes it takes up to 30 seconds or even more. I can't figure out what makes it behave slower, it seems to be completely random. I do notice that on some days most images will update fast, and only a few will take a long time. And sometimes it's the exact opposite.

Does anyone have any tips on how to remedy this problem? All the PSDs are 8-bit RGB with the same color profile, I try to keep everything very clean, no cluttering of layers etc, all the files are on the same drive, in the same folder actually, and I make sure to have enough space on all scratch disks etc. etc...

P.S. Just to clarify, I am not updating all the mockups at once. I am doing it one by one, so it's either just one or 2 smart objects updating at a time.