Probably the color TIFF LZW (compressed) image is compressed in strips, with each strip of separate compression containing a few lines. Probably the software you used to convert the color TIFF to the grayscale TIFF LZW compressed the entire image together, resulting in a single strip containing the whole image. In such case, the TatukGIS software must decompress whole image to display any portion of the image.
Try recreating the TIFF LZW grayscale image using different settings, or perform the conversion using a different software program.
We suggest using the TatukGIS Editor to convert from TIFF LZW color to TIFF LZW grayscale. The Editor creates the TIFF LZW image using a tiling method which can be particularly fast.