When is the phrase "get the lead out of one's pants used?  Is it not often used in daily conversation?

It means to do, whatever it is your are doing, faster than you are doing it.  The main idea is that if you are running, having a load of weight will slow you down, so lose that weight (get the lead out) and start moving faster.

it means you are going too slow so speed it up.  If you had lead in your pants that would slow you down., 

It's used reasonably often-- just "get the lead out". It means to get up and do something, or do it faster than you are.  It's even the name of a music group!

It's not used a lot, but it is used and people know what it means. It's usually used as an exhortation: "It's getting late. Let's get the lead out."

no, it is not often used.  it's an old saying ... meaning speed up.  much more prevalent when many first world men worked in heavy industry