Large file workflow strategy

A good example of this sort of problem, is Bert Monroy's Times Square illustration.  I don't know the actual pixel size, but going from this (note the green rectangle).

To this (the detail in the green rectangle) it must be immense!

It was finished eight years ago, so Bert's hardware would have been less powerful than Andrew's.

Here are some details from the above link:

Times Square

This is the largest image I have ever created, pushing the boundaries of the software and hardware as far as they can go. It was unveiled at the Photo Plus Expo in New York on October 28, 2010 as a work in progress. A 25 foot light box was constructed to display the piece that has been printed on a new material being introduced by Epson.
• The image size is 60 inches by 300 inches.
• The flattened file weighs in at 6.52 Gigabytes.
• It took four years to create.
• The painting is comprised of almost fifteen thousand individual Photoshop and Illustrator files.
• Taking a cumulative total of all the files, the overall image contains over 700,000 layers.

Bert worked on individual areas and flattened each of the 15,000 PSD files before incorporating it into the master document.  The way I do this to make Smart Objects which I double click to open in a new window as PSB files, and save that PSB file.  I then flatten the SO in the master document.  You still need to restart Photoshop now and again to consolidate the flattened SOs.  You can keep track of this by watching the size of the Photoshop Temp file in the root directory of the Scratch drive.

If you need to update an element later, then load the relevant PSB file. Edit and composite back to the master document.  Temporarily setting the layer blend mode to Difference will help you get perfect positioning.

I also add Copy Merged layers at the top of the stack (Shift Ctrl Alt E) and duplicate to a new document.  Save and close the original multilayered document, and check the scratch drive to see if Photoshop has relinquished the large Temp file.  Note: if you  are prone to the odd crash while using Photoshop, it is advisable to check the scratch drive now again for orphaned Temp files.   I don't have a problem with my fairly new 7900X system, but my previous, six year old 3930K system would often leave tens of gigabytes of such files.  If it does not have today's date on it, get rid.

That's a big file, but not that big. It shouldn't cause any problems.

Where do you have Photoshop's scratch disk? With such large files, all the action will be there. RAM can only hold a small fraction of it at any time. So you will have continuous read and write to the scratch disk.

Generally you get the best performance with scratch set to your system SSD. But then you need to make sure you have enough space. Scratch files can be huge, ten to hundred times the size of the file itself.

If you run out of scratch space, and you don't have a secondary disk assigned, the whole system might start paging, and that's when it really slows down.