5GHz routers are "faster" because of how many spatial streams it can support. You've seen the term MIMO used before (Multi-Input-Multi-Output), and that's how many individual data/spatial streams the chipset can support at 5GHz speeds. Some devices, like my Galaxy S7, are 2x2 MIMO capable on 5GHz networks.
Thanks to the IEEE, the 802.11 standard is a complete mess when it comes to advertising features to consumers. MIMO makes even less sense to consumers. I'm a network engineer and even I'm annoyed by it.
Some terms to note:
Single-radio dual-band: One radio, two bands, it's an either/or situation, shared bandwidth
Single-radio dual-concurrent: One radio, two bands simultaneously, shared bandwidth
Dual-radio dual-concurrent: Two radios, two bands, separate bandwidth
Blergh by Netgear with pictures:
https://www.netgear.com/landing/dual-band.aspx
So, this is the TP-Link AC900. The "900" in the model name is the maximum theoretical wireless link speed you could get out of it. But it's actually a AC750 underneath with some firmware tweaks.
The AC900 is a single-radio dual-concurrent design, which means that there is one software-defined radio available to run the WiFi networks. That single radio is designed to negotiate signaling for 2.4GHz and 5.0GHz devices at the same time. The Qualcomm Atheros QCA9563 chipset can drive a single 1Gbps connection to 900Mbps average Tx/Rx speeds, and it's basically allocating 1Gbps to drive both radios (minus some overhead).
This is useful if you have a mixture of WiFi devices and want to share as much of a 1Gbps network connection as fairly as possible. Having to choose between either 2.4Ghz or 5GHz networks doesn't help if you don't have interference from other networks, but would like to separate your devices otherwise.
However, there is a downside. By sharing a radio, there are fewer available data streams for clients on either network, so you're limited by how much bandwidth the chip offers for those data streams. If the 5.0GHz network was using the 80Mhz channel with a 400ns delay (or guard interval), the maximum amount of throughput you could achieve in lab conditions is 433Mbps.
Look at the table here, that's exactly matching the advertised speeds:
en.wikipedia.org
If this had two data streams for the 5GHz band, then bandwidth for that network jumps to a theoretical 867Mbps. But that is just two data streams, it doesn't mean that it is MIMO capable. MIMO would allow you to combine them on the client device.
Well, TP-Link isn't exactly lying here. but there are caveats. There are three wired antennae on this thing, and one radio is driving both bands at the same time. For this unit, think of it as having three data streams, one per antenna (more expensive devices have more data streams packed into a single antenna). There is one available data stream for the 5GHz network when active, which means there are two remaining for the 2.4GHz streams. TP-Link's spec table highlights this fact:
WiFi Speeds AC750
5 GHz: 433 Mbps (802.11ac)
2.4 GHz: 300 Mbps (802.11n)
Look at this table:
en.wikipedia.org
MCS 30 on that chart lists a max throughput of 300Mbps on a 2.4GHz network with two data streams. If you had two 2.4GHz devices, they would receive 150Mbps bandwidth each (which is another marketing blunder, it is total cumulative bandwidth, so 75Mbps up/down).
If you had to disable the 5GHz network on this router, you could have three data streams, which yields 450Mbps on the 40Mhz band with a 400ns delay (MCS 23). That's 150Mbps for three devices.
Now if you had a laptop with MIMO support on the 2.4GHz band, then you could reasonably get 300Mbps out of it from this router. This router can't do beamforming, so there will be some weird things happening when you have multiple devices connected but only a few at a time actually transmitting data. It won't prioritise data streams according to which device actually needs bandwidth, it will just keep dividing it up.
Indeed, most 2.4GHz chipsets in laptops are only capable of 150Mbps of throughput because they only support a single data stream.
It is likely that some tweaking would help with this, but otherwise you probably have a router that isn't capable of beamforming and doesn't have a lot of spatial streams available, and your devices may not be configured optimally.
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