It could be that you haven't allocated Preferences > Scratch Disks properly, or you don't have enough free disk space. Photoshop likes lots of Scratch Disk space. And ... drives are cheap. :+)
too much I/O ?
i double check every time i run cs6, i ve a dedicated, 100% empty scratch disc, my external 256GB intel SSD via thunderbolt to my imac, so photoshop has plenty of fast scratch. i also disable all other apps like mail, safari etc.., when i start photoshop.
i use shortcut: "alt"-key + mouseclick into the mask to switch into the mask.
i presume, that 16bit composings with a lot of layers (30-50 layers) in 300dpi, adobeRGB colorspace, is too much I/O for photoshop cs6 and cc2014, as i can reproduce the slow switching into the mask on my other mac pro 2014 with 64GB RAM, too.
I'd suspect some plugin. This is not a general hardware shortcoming, what you have should be plenty.
You're not still using Configurator panels, by any chance? I had this with CS6 until I disabled it. Strangely, mainly the masks were affected, just as you describe.
i dont use custom config. panels.
i did not tweak or change anything in my cs6 or cc14 install on both macs.
i only use one plugin on both:" Imagenomic portraiture"
Try turning off that plugin and see if things return to normal.