I purchased a low cost STM32F303K8 Nucleo board in the AVR Nano form factor. Ported my home brew gcode firmware. After a week of setbacks it is working. No visible difference in results from the AVR but the processing speed (72MHz) and the extra RAM (12KB) make for a more comfortable processing environment.
Have started to do some performance measurements. I currently drive the stepper motor update at 40HKz (25 usec). The AVR uses about 14usec and the ST about 6usec. Starting to instrument the time it takes to process a single G01 Xxx.xxx Yyy.yyy command. The AVR was about 1.6msec with the processor idle which goes to about 4msec when actively etching.
If you are wondering if this matters the answer is probably NO for cutting and YES for etching. The faster the processing the shorter the individual etch segments can get. User Beikeland posted an interesting unit (mm/hz) that I want to investigate.
I still need to find the magic recipe to enable hardware floating point.