Heavy:
* As heavy as lead (This is the classic and most direct comparison)
* As heavy as a rock
* As heavy as a millstone
* As heavy as a ton of bricks
Slow and Immovable:
* As slow as molasses in January
* As slow as a glacier
* As sluggish as a turtle
* As immovable as a mountain
Dull and lifeless:
* As lifeless as a statue
* As dull as a knife
* As boring as watching paint dry
* As colorless as a shadow
Other:
* As cold as lead (This compares to the temperature of lead)
* As dense as lead (This compares to the density of lead)
The best simile will depend on the specific context of your sentence and what aspect of lead you want to emphasize.