I can confirm that purging everything rarely actually purges much. As far as my experience goes most of the data is kept in the RAM for all open documents. What I also observed many times is that the purge results inside the same file and close to each other time-wise can be very different. For example, I have a file that takes up 45GB of RAM after some time working in it. When I purge all, it releases from 500MB to 20GB and I cannot explain yet why most of the time it is on the lower end. At times I am able to get a little better purge results if I create a new document while the big one is still open, add a layer, delete the layer and then use "Purge All" in that document. Or when I do a small operation after purging and then purge again. Could be completely random, too, though. Fast scratch disks are definitely the way to go no matter what, since 60-100GB are easily reached with large PSBs. Regarding general performance, I am still searching for the best combination of specs for PSBs with houndreds of layers and adjustments, some smart objects and at resolutions of 5K to 10K for daily matte painting. I upgraded two years ago from: i7 2600k@4GHz 64GB DDR3 GTX560 3GB to: i7 6600k@4.2GHz 64GB DDR4 GTX1070 6GB and all I can say is that it feels absolutely identical In general I think GHz per core, RAM size and speed and drive speed are the most important factors. So, to come back to the main question @OP: I do not believe you will feel a noticeable difference upgrading your graphics card. If the mixer brush is multi threaded, more CPU cores might help, but maybe it already boosts your performance to use a very fast scratch disk in case PS is constantly caching there while using the mixer brush (not sure if that's the case). Adjustment layers also decrease Photoshops performance drastically which is why I am not surprised to hear about the mixer brush lags set to use all layers:0
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