I guess the UART interface is imagined as possible interface between the µC and a WiFi module, ESP8266 might use it. There faster interface options, e.g. high speed SPI. But as a matter of fact, it's demanding to achieve higher throughput than a few 10 Mbps with a standard microcontroller, even with built-in ethernet PHY. It's not particularly a problem of interface speed, execution of the TCPIP stack is the bottleneck.
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Besides the Espressif controllers (ESP32 is the most recent device, faster than ESP8266), there are ARM based router chip sets.