slow performance vs img2sixel

[Alsan]

when the image size is small, almost the same between gosr and img2sixel, but when the image size is large, ie: 2560x1920, gosr and img2sixel are 4.44s and 04s respectively.

i use the following two commands for test:

cat /tmp/t.png | time gsor -
img2sixel /tmp/t.png