The arduino UNO or Mega2560 both run at 16mhz so same raster speed. Although you could change the xtal to 20mhz and recompile for the higher speed. They work fine for vector cutting but as you noticed, rasters can be slow. The 32bit controllers (lpc1769 for smoothie/grbl and STM32 for grbl) will run raster 3-4x faster. Depends on DPI density of the image.
https://embeddedtronicsblog.wordpress.com/
If you want to move up to a 32bit board.
This is my various 32bit firmware ports of grbl, smoothie, stm32, ESP32 that can be used on a laser.