Thus I suggest we can use: lzo1x_1_15_compress instead of lzo1x_999_compress. I tested myself, it is much faster to compress and the output file size is even smaller.
(Well I did a quick hack by let mksquashfs read the environment GZIP_LEVEL and use it like this in gzip_wrapper.c
char * gzlevel = getenv("GZIP_LEVEL"); if (! gzlevel) gzlevel = "4"; res = deflateInit(stream, atoi(gzlevel));
but it should be a permanent option. I had a quick look of how it should be implemented but find it hard to implement as it talk to a generic wrapper - and it handles it a bit complex to me)