Most Efficient Way to Create Thumbnails?

I have a huge volume of thumbnailing to do. Currently, I am using ImageMagick, but it's proving too inefficient (it's too slow, uses too much CPU/memory, etc.).

I have started to evaluate GraphicsMagick, which I expected to get "wow" results from. I didn't get them. Can someone take a quick look at my benchmark script (does simple speed and file size comparison only; no CPU and memory checks yet):

http://pastebin.com/2gP7Eaxc

Here's a sample output I got:

'gm convert' took 75.0039 seconds to execute 10 iteration(s).
'convert' took 83.1421 seconds to execute 10 iteration(s).
Average filesize of gm convert: 144,588 bytes.
Average filesize of convert: 81,194 bytes. 

GraphicsMagick is not that much faster -- and the outputted file sizes are SIGNIFICANTLY higher than ImageMagick.