Quite a few people, myself included, have had this question in mind, but the internet is not very forthcoming in answer. We all know that Instant On promises a choice between fast resume from sleep under its high performance mode or extended battery life during sleep under the battery saving mode., but Asus never explained how these promises are fulfilled.

After some testing, I can say with certainty that Instant On is simply Asus tagging its name on Intel’s Rapid Start Technology, which is a motherboard feature that allows the computer to go from sleep to hibernation without the operating system knowing.

Why is this useful, you might ask, given that OS’s are already capable of such a switch? The usefulness lies in the ability to save the hibernation file in a disk separate from the one the OS is on, something that Windows certainly cannot do on its own. With this ability the hibernation file can be saved on a small SSD while the OS resides on a large HDD, which allows for much faster resume from hibernation.

So what exactly are the high performance and battery saving modes in Instant On? The difference lies in whether the computer will go into hibernation from sleep after a certain amount of time. In high performance mode, the computer only enters hibernation if battery is critically low. Below I launch the IRST Manager to show the settings Instant On applied:

In battery saving mode, it does so after a set interval. While the IRST manager is showing an interval of 15 minutes, I am not sure whther that is in fact the default Asus has since that was the value I set with IRST before I install Instant On. Asus might very well be assuming a default interval of 0.

So how useful is Instant On/IRST? That depends on the type of hard drive you have.

• For computers with only HDD the feature is useless, as the hibernation file needs to be written to the slow HDD anyway.
• For computers with large HDD and small integrated SSD, it is very useful. While OS hibernation could take 30+ seconds to resume, IRST only takes 10 seconds.
• For computers with modern large SSD, the feature is once again useless. Base on my testing, both OS hibernation and IRST take around 10 seconds to resume. While IRST is the faster one—and it is only faster if you switch the hibernation partition from the slow integrated SSD to the main SSD—the difference is typically within a fraction of a second.

Now that I have talked about the benefit of IRST over OS hibernation, I should mention that it also has a considerable drawback: SSD wear. Typically IRST is paired with an integrated SSD partition that is exactly the size of the computer’s RAM. What this means is that the same sectors of the integrated SSD are being written over completely every time the computer enter hibernation, making it more likely to wear out and requires expensive service repair. Yours truly experienced this first hand when his Zenbook UX32VD’s integrated SSD malfunctioned just after half a year of use, which was only resolved after a warranty service.