Thunderbolt 3 ports are closer to PCIe 3.0 x4, not x8. Regardless it won't cause a huge performance hit depending on your workload (about 5-10% slower in gaming

Thunderbolt 3 ports are closer to PCIe 3.0 x4, not x8. Regardless it won't cause a huge performance hit depending on your workload (about 5-10% slower in gaming benchmarks). But YMMV with applications that require more bandwidth.