sometimes i like to listen to metal at faster speeds. 135% is ideal i think. it makes it like playing a 33 rpm record at 45 rpm. it's a good test for heaviness.
i've heard that if your riff isn't heavy in standard tuning, it's not heavy. for doom metal, many guitars are tuned to baritone tuning, which is about a 4th down from standard. coincidentally, 135% speed pitches the music up by approximately a 4th. playing at a faster speed counteracts the heaviness of a slow tempo by speeding it up, as well as ny making the voices sound chipmunky
if your music is still heavy at a faster speed, and a higher pitch, you have music that is heavy