Graphics buffer: trying to "fade out" an uint8array / best (fastest) way to manipulate pixels

Hi,

I'm playing with an 8x8 neopixel matrix. I want to do a pixel slideshow by loading images out from a server over time. I'd love to have a fade to black transition before moving to the next image. Anyone knows what's the most performant way to do this? Right now I'm doing a setInterval that loops the uint8array buffer (buffer.forEach) and multiply each index value with a gradual value every tick. This is super slow tho.

Thanks!