Datamax Prodigy Operator's Manual page 83

You will find that printing fine images on the less expensive direct
thermal and thermal transfer media at the higher speeds can be tricky.
At one heat setting the image will fade and at the next higher heat setting
the image can bleed. This is because the reaction temperature of the
media is so high that at higher rates of speed, it cannot react quick
enough.
To print fine images at higher speed, media with lower reaction or
release temperatures are required. On the slower end of the print rate
settings, crisper images are possible because the media is not being
stretched beyond its limits.
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Using the Printer Page - 75