It's not more efficient because it heats the water slowly - as said that makes no difference.
It's more efficient because heating water is the most energy intensive process in the wash cycle, so if you reduce the amount of water used, you reduce the energy needed massively.
However, less water means longer periods of soaking and agitation are needed in order to get the clothes as clean. The longer cycles really do use less though - if you have a smart meter or one of those plug things that measures energy use this is really clear to see. It surprised me because I was also sceptical, assuming that spinning a big heavy load of washing would use much more electricity than heating up some water, but apparently not.
There is often a button somewhere in the cycle options, where the buttons are to select things like spin speed, water temp, add rinse etc, which will speed up the cycle using more water (and therefore power) but less time. This should not reduce the load capacity - check the manual. If you want to reduce energy use on a shorter cycle, choose a lower temperature. Often a lower temp shortened cycle is less energy intensive than a higher temp long cycle.
Using a designated "quick wash" will often give you poor cleaning results as those are meant for smaller loads and/or less dirty clothing.
Personally I don't mind a long cycle, it's not like it makes a difference to me unless I need to put several loads through in one day or the noise will be disturbing. I just stick it on and let it do its thing. Doesn't make much of a difference if it takes 1 hour or 4.
I agree the apps are pointless. The app on mine doesn't even let you change anything. You can download new cycles but only one at a time, and it will overwrite the rinse + spin cycle - which, WTF. Why wouldn't it override something more niche, like sportswear or duvets?? I didn't even bother to reinstall it when I got a new phone.