ER-X contains 256 Mb DDR3 RAM, 256 Mb SLC NAND flash and MT7621A SoC (2 core 4 threads) with built-in gigabit switch.
It can be almost matched in wired perfomance with Asus N56U B1, Buffalo WSR-1166DD, Buffalo WSR-1166DHP, Netgear R6220, Lenovo Newifi D1 (I tried them all) and many other routers build on MT7621 SoC.
The main difference between them is firmware (and ER-X firmware have more advanced feautures that may not be needed in home).
But with any firmware MT7621A easily get 100 Mbit/s.
With hardware acceleration (in latest ER-X firmwares) - 1 Gbit (930 mbit pure data).
In other side - ER-X smaller than consumer WiFi routers, looks better (metal case) and not contains electrolitic capacitors.