What's the benefit of upgrading to "Hyperdrive II" in Stellaris?

The tooltip for the upgrade hints at the answer (It takes less energy to initiate a jump, so gathering the requisite energy takes less time), but it does seem to be a bit cryptic. Per the Stellaris Wiki:

Research allows for the warmup time - normally 15 days - to be reduced by up to 50%.

Hyperdrives, like the other two (starting) FTL methods, have three levels of engines, and if the first starting one takes 15 days to recharge, that would put a 50% reduction at around 7.5 days for the Mk III. So I'd Imagine the Hyperdrive Mk II would clock in at around 11.25 days to charge between each jump (possibly plus or minus a bit of rounding).

Side note; though the text says it requires less energy, you'll soon find that each engine iteration requires you to equip your ship with an additional 5 energy from power generators than the previous engine version. (they require 5/10/15 energy capacity, respectively)