any way of speeding up convert/resize?

[magick]

In most cases, the only way to speed up ImageMagick is to run on a faster processor or add more memory or limit the size of the input image. The cost of processing of an image is directly related to the area of the input image. You can speed up JPEG thumbnail creation by adding the -size option to the command line (e.g. convert -size 220x220 image.jpg -resize 220x220 image.png). For other image types, use -thumbnail to create thumbnails for a modest speed-up. There is also a modest speed-up if you are using a recent version of ImageMagick. The current release is ImageMagick 6.2.6-3.

[anthony]

Could it be communication setup/send/shutdown times instead?

Communications are often much slower as they have to get timing issues correctly, delaying the results delivery.