NAS server slower 2xHDD vs faster 1xHDD

When you're asking people to evaluate purchasing options for you, you should probably link to the product yourself. Make it as easy as possible for people to see the precise products you're considering.

It all depends, are you going to use it as a storage "cloud" or...?

My personal feelings are that it is a waste to buy a NAS with one bay.

https://www.synology.com/en-us/products/compare/DS216/DS116/DS216j

i would choose DS216:

• hot-swap
• more RAID options (RAID 1 for mirroring => better security)
• additional USB 2.0

important data

Backup to a secure CDN (use an online backup service), your files can live on any number of mirrored raid drives with long-lived devices, but nothing will protect that from theft, fires, non-hardware-level corruption, accidental deletion, hurricanes, bad mojo, flooding, etc.

If you need a multimedia file server, WD Red is great, but you could get away with a Seagate POS and it wouldn't make much difference. Drives will fail.

Generally speaking I would definitely go for more drive slots, as you will eventually come to that point, where you need more space, and just adding a drive then should be very easy.
But in this case, the 2-bay model has way less ram and cpu power. I'd buy neither. Maybe you should look what other manufacturers have to offer.

Regarding HDDs: Buy the cheapest! WD Reds are good, but usually around 20% more expensive. I'm running 4 WD greens in my NAS just fine, though lots of people will tell you never to do this :). I think if you don't plan to set up a RAID, you could buy whatever you want.
Just one thing: When buying HDDs, ALWAYS check the warranty before opening the sealed bag! Some retailers do sell refurbished or OEM drives without telling you.

You're just thinking about this wrong... a NAS with 2 disks isn't 'slower' than a NAS with 1 disk. Heck, if you're just going to run a single drive or slap a load in as a JBOD (Just a Box of Disks) you might as well buy a USB 3 external drive and move that around - you're not getting any of the benefits and are paying for the privilege.

If you're using a 2+ drive NAS, you want to use RAID, either RAID 0 for speed (data is striped across drives, almost like graphic cards in SLI) or RAID 1 for redundancy - there's no slow-down doing the RAID, because it's done automatically by the device controller and/or software.

If it's a 4+ drive enclosure, you want to almost definitely be using RAID10, which is the good bits of 1 & 0.

If you can afford NAS rated drives, then go for it, as they have guts designed for constant use and high MTBF, but in a RAID 0 or 1 setup, you can get away with standard drives. I'd avoid the eco drives (WD Green) as they aggressively power themselves down to save power and it can cause some NAS devices to assume the drive has failed because it has to spin back up.

If money is a real issue, look at QNAP alongside Synology - almost identical kit under the skin, slightly cheaper, different UI, Synology has better support channels.

edit - re: the actual device performance, it all boils down to how you're using it. The CPU and RAM won't be an issue if you're just serving a file share to devices on your network - a single core CPU w. 512mb RAM will boot a bit slower than the dual-core CPU, but it'll only be an issue if you load up the NAS operating system with apps like a web server, docker, a VPN, anti-virus, Plex/Kodi or one of the many other things. Then, yes, it'll be as slow as molasses as more and more people use it.

All 3 devices you list are ARM-based Marvell devices, which sip power compared to x86 Atom-based devices, but can't do stuff like video transcoding (meaning if you want to run Plex, you either need everything in h264/aac or you need something else to be the Plex Server).