I need an MCU that can handle 4 TCP sockets and 4 UDP sockets active simultaneously. Can MQX support this if I use a K70? How fast is Ethernet on a K70? Also, the MCU will also need to drive a TFT, draw a GUI, and control a system. External flash memory will be needed for code space. Can a K70 handle all of this efficiently?
The K70 is as powerful as anything in its speed class. Its EMAC has checkum offloading and its DMA operation is flexible and efficient.
I use the K60 and K70 with 50 TCP and 50 UDP sockets operating at the same time with good performance (in its required context).
When running the TFT with the K70 there is little overhead when the software is not drawing since it is automomous (DMA between SDRAM and the TFT).
It is however not possible to give benchmark results unless the overall system is defined accurately, and various things depend also the overall design as well as what Ethernet connections are used for.
The best thing to do is set up a test representative for your environment - you will not get the performance of much faster devices but whether you get the efficiency and performance to meet your goals depends on what your goals are, which are not specifed in the question.