Here's an imagemagick example; over six minutes with xargs, under 20 seconds with parallel

  $ ls *.png |wc -l
  3580

  $ time ls|sed 's/\(.*\)\..*/\1/'|parallel convert {}.png {}.ppm
  ls --color  0.00s user 0.01s system 63% cpu 0.016 total
  sed 's/\(.*\)\..*/\1/'  0.01s user 0.00s system 39% cpu 0.025 total
  parallel convert {}.png {}.ppm  97.39s user 61.87s system 890% cpu 17.883 total

  $ time ls|sed 's/\(.*\)\..*/\1/'|xargs -I {} convert {}.png {}.ppm
  ls --color  0.01s user 0.00s system 63% cpu 0.016 total
  sed 's/\(.*\)\..*/\1/'  0.01s user 0.00s system 39% cpu 0.025 total
  xargs -I {} convert {}.png {}.ppm  93.08s user 47.88s system 38% cpu 6:10.88 total