DarkViper uses it Toriks does not use it

And I see no difference when I compare their runs

What is the advantage of iFruit during a run?

You can upgrade the protagonists' personal vehicles.

I used the example with these two people, because they are about the same level. And their time is much closer to each other. But one does what the other does not do

You're not guaranteed to save time with it, it also depends on how well you drive and if you crash etc.

IIRC, Toriks doesn't use the app because he simply prefers not using it. It's more of a preference than a huge strat to save lot of time I guess

If you have upgraded vehicles and drive well, of course you're gonna save time.

I would surely be better without using iFruit

The ifruit app saves time but not so much that new runners should worry about it. The ifruit app is not hard to use, it seems specifically designed to be easy to use. Most of the upgrades, at least the significant ones, are done when you are stationary and doing nothing else e.g prologue cutscene, friend request tv scene, water tower with trevor etc.