Interfaces & linear scale for LinuxCNC

If you have plenty of RAM, stuff can "live" in RAM, without HDD latency issues.

HDDs are still good.

"Yes, this is valid, but requires 2 or 4 sticks of RAM, same size, otherwise it defaults back to single channel."

Not exactly, the amount of RAM that is dual-channel for a given pair of DIMMs is twice the capacity of the smallest DIMM.

DIMM = Dual-Inline Memory Module (the gold contacts that connect the RAM to the motherboard are on both sides).

Some errors here, anything over 4Gb of RAM is wasted on Linuxcnc. (2gb is probably enough if you can buy it today!)
Disk access has no impact on latency.

A more extensive real time latency test suite is included as standard in Debian (and most distros): sudo apt install rt-tests
and nothing in it mentions HDD or SSD
wiki.linuxfoundation.org/realtime/docume...howto/tools/rt-tests

Never hurts to have some extra RAM though, for I/O buffering, cache, etc.

I have 16GB because I have an insane amount of RAM from old PCs.

What about the BSD latency command?