If the OP is planning on doing large file sequential reads and writes all day, then he will easily saturate 1 Gigabit LAN connection with just one decent 3.5" 7200 RPM hard drive. Saturating a 10 Gigabit connection would definitely take quite a lot more drives and RAID 0, which would be funny for a little bit, but suicidal
@Magnusch , can you tell us more about the use cases and your budget? Otherwise it's like shooting fish in a barrel. 10 Gigabit NAS are not cheap and I am pretty sure no one is making one with less than 4 drive bays. How much space and performance do you actually need?
Personally I am still in the dark ages and I don't have a Mac with Thunderbolt 3. I use a Promise Pegasus (? I think) 4 drive enclosure connected via Thunderbolt 2 to my trash can Mac Pro. It works very well, but Promise is more expensive than Akitio. I have not used Akitio before, but they have been around for a long time and I have only read good things about their stuff. The reason why I chose Promise over Akitio back in the day was purely because it's easier to buy from the Apple store and my company is on the simple VAT scheme in the UK. This means that I have to purchase stuff over £2000 on a single invoice to get VAT back and I bought something else as well to make up the total. If I was buying it personally then I would have gone for the cheaper Akitio.
A DAS will perform as fast as the hard drives can, depending on how many bays you have and whether you use them JBOD (just a bunch of drives) or in RAID. Bear in mind that everything we have discussed so far is using SATA connections. So if you are really needing performance, then you can of course buy SATA SSDs. These are faster than a HDD, but slower than a PCIe based SSD, like the one found in your iMac. Again though... unless you are copying > 10 GiB back and forth all day, it's unlikely you will notice this difference. Use case is important. For ultimate performance you can get a TB3 enclosure that will hold PCIe SSDs (up to gen 3). Of course as you move from HDD --> SATA SSD --> PCIe SSD than the price per TB increases a lot.