Can you post a pic of the settings used, the picture you were attempting, and the output? A picture is worth a thousand words, and all that - it helps to see what you’re working with.
130IPM at is about 55 mm/sec. 300DPI grayscale is about 12 dots per mm. At 55 mm/sec you’d have about 650 gcode instructions to process per second, and your 8-bit controller is capable of roughly half that, possibly a little less (I think 8-bit GRBL caps at about 250 to 300 gcode instructions per second). So, that could explain the performance hit.
GRBL will lower the power to compensate for the fact that it can’t hit the speed you asked for, and if the controller is set to use 0 to 255 as your spindle range, you were starting with 25% of that, meaning 0 to 63 possible shading values. If you’re only going half the requested speed, your number of unique shades is now down to 0 to 31.
Increasing the S-value range on the controller might help, as it would give a higher range of numbers, though I’m not sure if that controller can use them. The chip is capable of up to 16 bit PWM resolution, but it depends on which of the outputs they used and how it’s configured.