If his original TIFF images are fax compressed it may takes a bit longer to uncompress them than to decompress LZW, which might explain some of the performance difference. I know libtiff's group 3 compression code was fairly slow way back when I dealt regularly with fax.

You might want to try mixing scaling mode for decent performance, and I'd expect decent quality for scaling monochrome images too (Imager 0.54 +).