Why LP leak is more serious?
The rate that a tank empties is much more dependent on the orifice size than the pressure gradient.
The HP port on a regulator has a tiny orifice because it just has to transmit pressure, it doesn't have to supply gas. The LP port is much larger because it has to supply you with enough gas to breathe.
Here's a good example of that. Cut the HP hose, an AL 80 will drain in 22 minutes. Cut the LP hose, 81 seconds.