Fast screen dump via frame buffer in unix

I need to read an area of my screen about 15x15 pixels, such that the operation is fast, i.e. less than half a second. I have been using Imagemagick tools via import -screen -window root -crop 15x15+100+100 a.bmp , but I cannot get it under 2 secs - this is way too long for my needs. My screen is 24dpp at 1920x1200 resolution (and I need to use this resolution).

I wondered if there was some way of speeding it up e.g. by reading the memory directly via the frame buffer or otherwise and if you can help me with a command.

A guy on the imagemagick forum suggested something along the lines of

convert -size 1920x1200 rgb:/dev/fb0'[0]'  -channel RGB -separate -swap 0,2 -combine outfile.png

but it doesnt work - it gives me the error "convert: Unexpected end-of-file `/dev/fb0': Invalid argument."

Thanks

Victor