I've been running OMV on single board computers for a while now. I first started with a Rockpro64, then moved to an Odroid N2 (both ARM SBC's). I saw a decent improvement switching from the Rockpro64 to the N2, however I've started to build alot of things using the Odroid H2, which is an Intel J4105 CPU with dual gigabit NICs, an M.2 for NVME and uses SODIMMs for RAM.
Right now I have the 2 NICs bonded (rr balanced), OS is running on a 256gb NVME, and I have 8gb ram in dual channel. I know the bonded NICs don't improve single transfer speeds but they're already there and should help with multiple transfers. I also know the OS running on NVME probably isn't improving anything over running the OS on eMMC, but NVME is actually cheaper than eMMC these days. RAM, I don't know, I'm not sure what kind of use OMV makes out of ram, I have 16gb (2x8) readily available as well if there was some improvement to be had there.
My Storage is 3 USB3 4xHDD enclosures, it's just set up as JBOD, no raid or anything. Disks are all ext4 single partition mounted NFS shares. I'm not trying to build anything crazy, I just figure I have a few hardware options on hand an I'd like to get the most out of it. Like I could either partition the NVME or move the OS to eMMC (have a 16gb chip on hand) and use the NVME disk as some sort of cache, though from searching around I can't find anyone mention a beneficial way to use SSD cache. Any thoughts?