Many people have been complaining lately about performance issues on their SLI system. On both the NVIDIA subreddit and on the NVIDIA forums users have come to the conclusion that performance drops when SLI and G-SYNC are working together.
The guys over at ExtremeTech decided to test and see if this way true. They have confirmed that frame rates do fall if both features are enabled. The problem seems related to timing according to their data, but there is no clear solution yet.
These issues were quite big in titles like Rising Storm 2 (206 vs 75 FPS) and not as apparent in other games like Witcher 3 (113 vs 98 FPS). “Turning G-Sync on and using SLI is not guaranteed to tank your frame rate. […] Different games showed three different performance models”. In Deus Ex Mankind Divided the gap appeared only on DX11 mode. In Far Cry 5, the penalty size increases as the frame rate rises, and in Hitman the results were even more confusing.
There still is no clear explanation what the issue is. It does seem however this problem is not present on Turing, so if you have RTX cards you should be safe.
NVIDIA has not made any official comments on the issue, but hopefully this can be fixes with a driver / software update.