Total absence of speed is death.
I'd suggest "Total absence of *motion* is death."
Plus, those Lucretian citations seem to be discussing the smoothness or roughness of the atoms/seeds/particles themselves. Smooth atoms lead to easier, unimeded motion. Pain seems to be caused, per Lucretius, by jagged, rough atoms tearing and ripping as they move.
Sensation is motion. The cessation of motion is death.