Is there a way to speed this up? Tile matching

[Ofnuts]

If you do Filter>Blur>Pixellize you get tiles where the R, G, B channels are the average for the tile (so the same as the values you get from the histogram calls). If you Color>Desaturate that using the "Average" method the gray you obtain is therefore (R+G+B)/3 from which you can obtain R+G+B (if needed) (although it could be better to use the Luminosity, which at that point shouldn't be harder to work with).