Photoshop needs a long awaited major overhaul

If you want to post Feature Requests please mark them accordingly – »Idea« not »Discussion«. 

»Dicsussions« are basically just a user Forum. 

What kind of work are you doing that would need a Brush that is more than 90% of the largest side of the image anyway? 

When I swith to 256 GB RAM

By @Brecht Corbeel

The amount of RAM is actually not that important. Raster image editing requires huge amounts of memory, much more than any RAM you may have installed. No matter how much you have, it's not "enough".

That's why Photoshop uses a scratch disk, and that's the critical bottleneck. All that temporary data is written to disk, and you need to have enough free space on a fast disk to handle it. RAM is more of a cache to the scratch disk's main memory.

With your file sizes, I'd never be comfortable with less than at least 1 TB of free space on an NVMe drive. The scratch file needs to contain all your history states for all open files, plus overhead, which can be considerable for smart objects.

Aside from that, brush response is largely limited by your GPU and driver. A buggy or suboptimal driver can slow this down considerably, as can an underpowered card.

Generally, a lot of people don't understand the significance of raster data vs. vector data. Raster editing is orders of magnitude more I/O-intensive than vector editing. There's just an enormous amount of data that needs to be moved around.

Save times are determined more by data encoding than actual writing to disk. That's why you don't see that much difference between HDD and SSD. Make sure to disable PSB compression in Photoshop Preferences! This speeds up saving dramatically, like from 10 minutes down to, say, 40 seconds (depending on layer structure).

Sorry for the typos I cannot edit it seems.