Performance penalty seen when using NVIDIA G-SYNC in combination with SLI

The use of SLI is pretty rare and there aren't a whole lot of folks out there who use it in combination with G-SYNC. The latter technology is supposed to have no performance impact, but various reports are floating around on the web from users who experience a significant performance penalty when using G-SYNC in combination with SLI. ExtremeTech decided to put this to test and confirms there's indeed something strange going on with G-SYNC is enabled on a system with two NVIDIA cards.

Depending on the game settings and which API is used, there can indeed be a very significant performance drop:

It’s always dicey to try and test forum reports, not because forum commenters are liars, but because most don’t provide sufficient technical information to be sure you’re reproducing a problem correctly. I don’t have an explanation for this issue at the moment and I realize that “timing problem” is extremely vague. It’s a theory that happens to fit some common-sense facts. Our results collectively suggest that the issue is real and that the performance gaps could be as large as Nvidia users say, particularly if they continue to worsen as frame rate increases.

The exact reason why this occurs is unknown, and at the moment NVIDIA hasn't publicly acknowledged this issue. We agree with ExtremeTech that NVIDIA definitely needs to address this.