Large format document save time - dependent on CPU or drive speed?

On the new machine, internal storage is a lot more expensive than external. Would I see a difference in thunderbolt 4 vs thunderbolt 5 vs internal SSD? Can photoshop save quickly enough to take advantage of the higher speeds?

By @Jonathan Lipkin

No, when comes to Photoshop large document saving, you won’t see much difference among any of those options or any of the current Macs. It’s easy to see why: Open Activity Monitor, which is in the Utilities folder of every Mac. Start a very large Photoshop save, and in Activity Monitor, click the Disk tab and look at the readouts at the bottom of the window, especially Data Written/Sec. You will probably see that Photoshop is moving data at well below the expected transfer rate of the media you’re saving to. You might see a brief peak transfer rate of up to a few hundred MB/sec, which could saturate a good hard drive, but falls well short of how fast other modern storage can transfer data. The reported sustained/average transfer rate might be well under 100MB/sec. If those are the numbers you see during a Photoshop large document save, that means it isn’t even reaching a fraction of Thunderbolt 4 potential (up to 3000MB/sec), much less Thunderbolt 5 (up to 6000MB/sec), or Mac Studio internal storage (7500MB/sec or more). 

If you then switch to the Activity Monitor CPU tab while doing a large Photoshop save, you will probably find that Photoshop is using only a couple of CPU cores during a big save. Even the cheapest Macs come with 8 cores now, a base Mac Studio comes with almost twice that many. If you see a percentage, 100% equals one core, so maxing out the CPU on a current M4 Max should report 1400%. But when I save a PSD on my M1 Pro, Photoshop tops out at 200% or the equivalent of two cores, leaving the majority of CPU power idle.

What all that means is that if you want to speed up PSB file saving at this time, these are the many ways that throwing money at the problem mostly won’t work: 

• Upgrading the number of CPU cores doesn't help, because a Photoshop save only uses around 2 of them and all current Macs already have at least 8 CPU cores (most have 10 cores now).
• Upgrading the CPU generation should help a little, because single-core speed increases with each generation (M1 to M2 to M3 to M4).
• Upgrading the number of GPU cores also won’t help, as far as I can tell. 
• Upgrading the amount of Unified Memory probably won’t help as long as the Memory Pressure graph is green in the Memory tab of Activity Monitor. If the graph is frequently orange or red during a Photoshop session, more unified memory is a good idea for overall stability, and might make saves faster if memory was a bottleneck before, but chances are more memory won’t make much difference in save times. 
• Upgrading storage speed won’t help, because if the transfer rate during a Photoshop PSB save doesn’t come close to maxing out the transfer rate of your current storage, it won’t make any difference to use newer, much faster storage. So if you want faster Photoshop saves, upgrading Mac expensive internal Mac storage is not going to do it. 

One explanation given for the low CPU usage is that there are apparently aspects of saving that can’t easily be parallelized because they depend on other data being rendered first, so it has to be done in a certain order. An analogy is how you can’t speed up a pregnancy by adding more mothers.

If some day they find a way for a Photoshop save to use a lot more of the CPU or GPU cores, they might be able to increase the transfer rate of a big save, and then maybe hardware upgrades would speed up Photoshop large document saves a lot more. But right now hardware upgrades make very little difference in this area. 

Compression is all about encoding. That's CPU. That's what takes time. The actual save to disk is a tiny fraction of the total time.

This is easily verified by monitoring CPU and disk activity in real time (I see you're on Mac, but on Windows this is easily done in Task Manager).

PSB compression is unbearably slow, like, say, three minutes versus 15 seconds. The algorithms are old and probably written for the small files that were the standard many years ago. I don't think it can take advantage of more than at most a couple of CPU cores. But note that this may not necessarily reflect the age of the algorithm - some things just have to be done sequentially.

So I'd say the real answer to your question is "turn off compression". Then decide not only how much drive space you need now, but what the strategy should be for expanding drive space as you need it. Because you will run out sooner or later.

A Thunderbolt 5 RAID will be very fast but also pricey. I'd prioritize RAM first, then internal storage, then CPU cores.

To get an idea of pricing, check OWC:

https://eshop.macsales.com/shop/external-storage

https://eshop.macsales.com/shop/thunderbolt/thunderbolt-expansion