We installed an NVIDIA GeForce GTX 1060 in the Sonnet eGFX Breakaway Box, and ran the Heaven benchmark with Quality: Ultra; Tessellation: Extreme; and Anti-Aliasing: x8; on a 1080p monitor from all three Thunderbolt generations.
The results were:
Thunderbolt 3 (40Gbps) - 55.3 fps
Thunderbolt 2 (20Gbps) - 54.3 fps
Thunderbolt 1 (10Gbps) - 51.8 fps
Results may vary by application and graphics card.