I heard a lot that Gigabyte's Xtreme Engine is a very bad software, but my question is why? I ordered a new gaming PC with a GTX 1070 G1 Gaming and I want to know how much performance I lose by using Gigabyte's software for OC.
You shouldn't lose any performance assuming you are applying the same OC in either program. I personally don't like gigabytes software because of it's UI and lack of functionality versus afterbuner, afterburners monitoring software is far superior in my opinion and it comes with rivatuner which is good for in game overlays or capping framerate. However in terms of applying an overclock they both work fine. MSI afterbuner is just more mature, gigabyte will likely keep updating their software and I would expect it to be just as good eventually.
Gigabyte Xtreme Engine is extremely laggy. Did you notice the buffering circle that goes on for like 10 seconds every time you change your input clock speed? This gets very frustrating when doing incremental change. The whole thing is just slow and very laggy. The same is true for Aorus Engine (which comes with the 20x series).
On the other hand, MSI Afterburner is so fast. Having used both, I can tell you it is a WORLD of difference. And as Dunlop mentioned above, msi ab just has superior UI and many other features, including Riva tuner which is a superior OSD.
You probably can't find a better OSD than msi ab's riva tuner. ASUS also has its own gpu overclocking utility called GPU Tweak II, which I have used and can tell it is equally fast as the MSI Ab. However, the OSD that came with asus GPU Tweak is extremely buggy, causing game lag and fps drop.
So yea, MSI AfterBurner is the one to go after.
If I was to rate gpu overclocking software, it would go like this (1 being the absolute best, and 10 being the worstttttttttttttt)
MSI AfterBurner (1)
ASUS GPU Tweak II (2)
Evga Precision (3)
Gigabyte Aorus Engine/ExtremeEngine (1000)