I've just spent a weekend reading reviews and opinions about all these prosumer NAS offers, only to conclude how large a waste they are, and I mean any product that has 1Gbe Ethernet as the primary connection.
Even if you buy the smallest, 2-bay NAS, you might want to stick 2x6TB Seagate Enterprise drives in it for maximum space and performance (ops!). Except that those drives coupled can yield over 400MB/s in RAID 0 in read speeds. That requires 4Gbe in network traffic, of which you will get zilch.
In all, it doesn't make sense to buy a NAS today that doesn't have 10Gbe in it. You would be effectively castrating all the hard drives you put in there.
And as far as I can see, Synology doesn't have good products in that area at all. All their consumer-oriented products are 1Gbe only.
QNAP, on another hand, offers plenty of products with 10Gbe on-board, starting with TS-x70, PRO and all the way to TS-EC1080 Pro.
They of course would cost more than those stupid 1Gbe "NAS" from Synology, bit you will get a proper server.
If those are too expensive, you will be better off with an all-in-one USB 3.0 external hard drive, with couple drives in it in RAID 0, which can give much faster speeds than the pathetic 90MB/s with those Synology 1Gbe NAS products.
Also, in terms of the value, QNAP TS-870 PRO is definitely at the top. That's a whopping 8-bay monster, with 2 x 10Gbe ports. And with a proper search and QNAP's regular 30% discount one can be found for around 1K, now that's one awesome investment!