GoKawiilA Thunderbolt 4 port carries 40Gbps, and Intel's spec allots PCIe data 32 Gbps of it, an eighth of the bandwidth a desktop PCIe 4.0 x16 slot provides. Most of what the card in an eGPU enclosure does will never cross the Thunderbolt cable. Textures and shaders get loaded into VRAM once and remain there, meaning the cable only gets busy when a game pulls in new assets or sends finished frames back to the display.
Portables review site Notebookcheck tested an RTX 2080 Ti enclosure back in 2021 and found deficits ranging from seven to 32 percent when compared to a desktop. Losses did shrink to single digits at 4K on an external monitor, but rendering to a laptop is slower because finished frames travel back down the same cable. In contrast, PC Gamer tested an RTX 4070 Ti and 4090 over Thunderbolt, OCuLink and desktop PCIe. Its RTX 4070 Ti ran around 25 percent slower over Thunderbolt than OCuLink in many 1440p tests, with lower 1 percent lows and more stuttering.
PC Gamer also claimed that upgrading from the 4070 Ti to the 4090 delivered smaller gains over Thunderbolt and OCuLink compared to desktop. So, the faster the card, the more of its performance the Thunderbolt cable restricts, making a mid-range GPU the more sensible choice for an eGPU enclosure. Thunderbolt 5 provides 80 Gbps of connection bandwidth, while Razer's $350 Core X V2 connects the GPU through PCIe 4.0 x4, nominally 64 Gbps. That gives an x16 graphics card four lanes instead of the 16 available in a suitable desktop slot.