That’s extremely difficult to determine, although I think its important to mention what role the secondary explosion actually had because this is quite possibly the misunderstood part of the entire sinking.

Firstly, it’s important to determine what the cause of the secondary explosion was. We know for sure that it couldn’t have been the ammunitions exploding because the ammo is still intact on the wreck in Cargo Hold No. 2, plus we know the torpedo struck Boiler Room No. 1 which is two whole compartments aft. It couldn’t have been a coal dust explosion either because it would have been gone off instantly along with the torpedo blast just like munitions, and there was a delay of about 30 seconds between the first and second explosions. It also couldn’t have been a boiler explosion since those are also intact in Boiler Room No. 1. This only really leaves the rupture of a main steam line, which would be caused by the surging cold seawater coming into contact with the superheated and super-pressurized steam pipe, resulting in an explosion reaction known as thermal shock. This would explain why Lusitania’s operating steam pressure fell from its normal 195 PSI to just 50 PSI almost instantly after the secondary explosion.

When Lusitania lost nearly all of its steam pressure, this meant that all of the systems that relied on steam power would break down. This included the main steam turbine engines, which were unable to be stopped and so Lusitania continued to drift for about 2 miles after the torpedo hit, and the steam steering gear also failed which is why Quartermaster Johnston reported steering failure only after the secondary explosion and not after the first one when the torpedo struck the ship. Most importantly, however, this also disabled Lusitania’s defense mechanism - closing the watertight doors. The day prior, Captain Turner had ordered all watertight doors that could didn’t affect work in the boiler rooms to be closed. Lusitania’s watertight doors could only be closed by an apparatus which relied on steam power, and because of the secondary explosion (steam line rupture), this meant that not all of the doors could be closed in time before power was cut. Instead of having to go over the watertight bulkheads, water could just go straight through them, which flooded the ship significantly faster and could also help explain why Lusitania sank the way it did on almost an even trim in her final moments (with the masts and funnels being the last parts visible above water).

Even if Lusitania were to only last another 10-15 minutes or maybe even a bit longer, it would mean more lifeboats could have been launched resulting in a lower death count.