NEXRAD preview PNGs are 12200 x 5400 pixels. This takes over 650 MB of RAM to load each frame.

Problem

Normally, you should be able to control ImageMagick use of RAM with environment variables:

• MAGICK_MEMORY_LIMIT Amount of RAM heap to use
• MAGICK_MAP_LIMIT Amount of RAM to use before paging to disk
• MAGICK_DISK_LIMIT A very slow last resort, before crashing.

I tried commands like

MAGICK_MAP_LIMIT=2GB MAGICK_MEMORY_LIMIT=2GB

but they still paged to disk, taking 1000x longer even with SSD and wearing out the SSD with writes.

MAGICK_DISK_LIMIT=100MB mogrify -scale 10% "*.png"

would give error message as expected.

I filed an ImageMagick bug report.

Workaround

For Linux, there is a RAM drive available at /dev/shm or /run/shm, which is not available on Windows, even with Cygwin or Windows Subsystem for Linux.

I force ImageMagick to use RAM drive, which gave me 1000x speedup by:

MAGICK_TEMPORARY_PATH=/run/shm mogrify -scale 10% "*.png"