Theoretical Max SPI Bus speed on CPU Serial pins and availability through OS

Hey!

I'm planning on using the SPI Serial connections on the LP 3 Delta, however instead of messing with the arduino side I want to use the CPU serial pins above the arduino pins to avoid speed limitations. To note I haven't found any docs yet that talks about the max speed or the availability of those specific pins through an OS (Specifically Linux or Ubuntu, I have not yet gotten to that part yet though so I can't say which version of either I am going to be using), so could someone perhaps help me find those details?
For ref. I'm planning on having an MCU relay info back to the LP 3 Delta while the LP sends back commands and other configs to the MCU as the LP is going to be the main processor for my robot, but I would rather have the MCU take most of the sensor reading load off of the LP and just relay the important stuff, which in this case involves a high speed bus, preferably SPI since those are available on the connectors. Also included a png of which pins I meant.