Tomb Raider debuted for Windows in 2013 and developed by Crystal Dynamics. Tomb Raider only came to Linux in 2016 and was another port carried out by Feral Interactive.
It's probably no surprise at this point with this game being another independently-ported title, the Linux performance even with NVIDIA proprietary graphics were much lower than on Windows 10. The GeForce GTX 1060 was at 61% the performance of Windows 10 while the GTX 1080 was at 56%.
At least when switching over to the ultimate image quality settings, the results were much closer between Windows 10 and Ubuntu Linux for the NVIDIA Pascal graphics.