Faster ADC Speeds / Multiple ADC access

I'm trying to figure out how to get faster read speeds from the Portenta than what AnalogRead() provides. I have tried accessing the registers directly to accomplish this, but I am unable to do so from within the Arduino IDE. Any help on either of these fronts would be appreciated.

On a related note, AnalogRead() does not let me pick which ADC to use. Is this just not a feature of the Arduino driver?

Analog read

analogRead(pin)

If you supply a pin number you can read differing AD outputs.

I appreciate the advice, but I specified in my original post that analog read is not reading fast enough for my requirements

What is fast enough?

You could look up the specs for the A:D on the Uno and figure out how fast can you go.

Most likely the MCU A:D's won't be fast enough, get an external A:D, spend those bucks and get one fast enough.