Thanks for that info Michael, helps a bit.
From my understanding that ASUS 4x M.2 add on card will work on your motherboard if you want more internal storage space within the PC itself.
What other specs for your PC?
What storage config?
ie. do you have M.2 storage? if so, one or two? What model SSDs?
Do you use plain old SATA drives? if so, SSD or HDD?
Can we assume that you have backup storage available for your images?
eg. I have a NAS(QNAP) where I archive my images, but still have all those backed up images available on a HDD in the PC too.
Plain old simple redundancy.
Because the NAS is over a Gb Ethernet connection, it's not particularly fast(access) and all programs slow down when trying to access the files on the NAS.
So a relatively fast (like a cache) storage space makes file access quicker for using those files on the PC. Once edited, back them up to the NAS for data integrity.
if you have this setup, then it's not a totally mad idea that if you create a RAID0 internal storage space in the PC for the everyday use of your files .. will speed up file access to them.
RAID0 is not a totally secure way to 'archive' files .. if it fails in any way you lose all data .. no redundancy.
Of course there are other RAID methods that can also help access speed, but you will be looking to balance space vs speed .. etc.
I used to have a RAID0 config on a very old PC, but back in the day before SSDs were a real thing for us mortals. Old slow drives used to benefit from RAID0. But it was only a setup for speed, not backup.
Just my experience but I just don't trust RAID at all any more, so just keep multiple backups and do it semi manually.
I'm not entirely sure that RAID0 on a set of very fast M.2 drives may produce any real benefit either tho(never done it). But on plain jane(ie. slower) SATA SSDs it can help a bit. Benefit wasn't worth it tho.
Those ADATA M.2 XPG SSDs do have amazingly fast read/write times(BTW, your primary concern here is read times going by your OP). Write speeds are usually also tied to read speeds tho, so one will go hand in hand with the other.
I have two 400-500MB/s capable SSDs in my PC, connected to SATA. configured in RAID0 they give 600-700MB/s, faster for sure, but the benefit isn't worth the effort.
My M.2(Samsung) isn't as fast as the ADATA M.2 I got for my sons PC. The motherboard limits it's speed to a degree. My PC is now about 5+ years old tho. My sons was built last year, so it's newer tech.
Where the ADATA XPG SSD tops out at high 1800MB/s on my motherboard, on his newer motherboard it reaches 3000-3200MB/s easily(all read speeds). Write speeds are a lot slower at about 1500MB/s and 2500MB/s respectively!
What this shows is that just spending big $s on the fastest hardware doesn't necessarily equate to faster hardware .. remember the comment that the weakest(or slowest) link in the chain will be the limiting factor.
eg. lets say you decide to get yourself an external M.2 enclosure and connect it via Thunderbolt/USB-C. You can easily get 3000+MB/s transfer rates from a M.2 drive. Thunderbolt(on the PC) may give you about 2000MB/s transfer rates.
Thunderbolt(on the external enclosure) may only give you 1000MB/s tho. Remember the point I highlight that Bob Foster first noted ... not all chips are created equal. Each device has it's own chipset(interface) device that allows communication(data transfer) between the various devices. This is just a hypothetical eg. and to be sure .. 1000MB/s storage transfer rates is not slow, and would(or should) never slow down your preview experience of your images the way you described in your OP.
The process I described that I do in XnViewMP never causes me concern.
Have you done any storage speed testing?
There are many free programs out there, one I use for all storage testing(easy to read/use/understand) is Crystal Diskmark. You don't need to install it(which I hate to do uncessesarily) .. just run it once you unzip the download.
It then outputs a text that easy to understand.
eg.
CrystalDiskMark 5.1.2 x64 (C) 2007-2016 hiyohiyo
Crystal Dew World : http://crystalmark.info/
-----------------------------------------------------------------------
* MB/s = 1,000,000 bytes/s [SATA/600 = 600,000,000 bytes/s]
* KB = 1000 bytes, KiB = 1024 bytes
Sequential Read (Q= 32,T= 1) : 1667.959 MB/s
Sequential Write (Q= 32,T= 1) : 969.903 MB/s
Random Read 4KiB (Q= 32,T= 1) : 265.704 MB/s [ 64869.1 IOPS]
Random Write 4KiB (Q= 32,T= 1) : 200.334 MB/s [ 48909.7 IOPS]
Sequential Read (T= 1) : 1400.098 MB/s
Sequential Write (T= 1) : 963.801 MB/s
Random Read 4KiB (Q= 1,T= 1) : 48.157 MB/s [ 11757.1 IOPS]
Random Write 4KiB (Q= 1,T= 1) : 122.435 MB/s [ 29891.4 IOPS]
Test : 1024 MiB [C: 14.3% (33.9/237.9 GiB)] (x5) [Interval=5 sec]
Date : 2016/10/02 0:30:48
OS : Windows 10 Professional [10.0 Build 10586] (x64)it just gives an idea of how fast your storage devices are operating in your system.