I'm trying to select a router to optimise the amount of throughput that the router can achieve, and was wondering what were the main hardware factors are that determine wireless throughput. Should I be looking for certain types of wireless chips, or higher RAM on the router?
The reason I ask is that any router that supports 802.11ac claims to offer 1200Mbps, but I've tested different AC routers and they all have different speeds in testing.
Well for starters
AC1200 WiFi is made up of 300 + 867 Mbps speeds
So your either going to get 300Mbps or 867Mbps for and indvidual client and less in practice
While you should look for 16 MB flash and 128 MB RAM at a minimum for new devices these days (for the simple reason to keep it supported long term), many factors contribute to real-life performance. Yes, the clockspeed matters, you do need a certain minimum to cope with certain demands, yes the number of cores matters (look at >=2, if you can), just as well as the architecture (mips vs arm) and even the generation of this target arch (ARM cortex A7 vs A9 vs A15 vs A53 vs A72, etc pp). But neither of these determine the expectable performance as a router alone, the SOC itself and its I/O capabilities matters just as well (how many CPU ports/ how are WAN and LAN connected to the SOC, how good are the SOCs ethernet and switch drivers, are there hardware acceleration engines and are they supported on OpenWrt), before looking at the way the wlan cards are connected (part of the SOC, PCIe, PCI, SDIO, USB, ...) and simply the quality of the wlan cards and the reliability of the free drivers.
I don't have a great summary link to link to, but do read the SmallNetBuilder guide, as well as the old FAQ (understanding that the numbers are outdated.)
In short, the number of spatial streams is the important one. The larger of the number of spatial streams in your AP and the number in your client yields peak performance at 0 range, and the more spatial streams the better for keeping that up at range. 4x4 ("AC2600," usually) devices max out a single client.
"Tri-band" routers are kind of like 2 APs in one, and only give more throughput if you have a ton of devices and they can split up between the radios.
Most other number-increasing tricks are meaningless. Unfortunately, you have to look at a table to translate from "AC3200" to "two 3x3 radios."
If you're trying to roughly predict what speeds you'll actually get on 5GHz, then take the better of your device and router (let's say a laptop with an "AC1200" 2x2 radio), take just the 5GHz part of that number (AC1200 = 300 + 867), take ~60% of that (867*.6=520), then understand that's with perfect signal, no interference, and taking up 4 channels. So I'd expect ~500mb/s in that scenario next to the router, maybe 250 at a reasonable distance, more like 150 or so with any other devices around. tl;dr the numbers are just marketing.
That's for wireless performance. If you only have wireless clients, or a slow ISP connection, you may be limited by those and not run into wired routing speed limits if you buy a decently fast modern device. If you have, say, 150mb from your ISP and want QoS, then you'll have to check to see if you have routing performance. @slh's answer deals with that.
@slh is spot on, there's no way to look at the hardware and tell if a router will achieve certain goals, but there's lots of ways to be pretty confident that it won't!
Gotchas that may not be evident also include:
Many router operations are effectively single-threaded don't utilize multiple cores very effectively. For those, a two-core CPU won't be 2x faster than a single-core CPU (but will let you do different things at the same time)
No matter how many Ethernet sockets there are on the back, there are typically only one or two "real" Ethernet connections inside. If throughput of more than a couple hundred mbps is important for you, you want two (or more).