|It is very cumbersome, thus slowing your movement.|
Ponderousness is an ego very occasionally found on plate armours, where it will be accompanied by a high enchantment to make the item not purely bad. The pondering ego raises your movement delay by 1 (most species have 10 by default). Generally, a plate armour of ponderousness is not worth wearing: even a Cheibriados character will prefer to get some sort of resistance in that slot, and the slower movement is not worth the moderate bit of extra AC for any other character.
Prior to 0.14, the movement delay penalty was 2.
Prior to 0.10, Cheibriados granted an ability that allowed you to convert any piece of armour into an artefact, maintaining all its former properties but adding ponderousness; in exchange, he would greatly increase your stats and provide some resistances, not to mention making the armour immune to acidic corrosion (as well as to further attempts to enchant it!).