Introduction

Today, we are testing the performance impact of PCI-Express Gen 3 on the entry-level Radeon RX 6400 graphics card, which we reviewed last week. Until a few generations ago, the notion that PCI-Express bandwidth is limiting the performance of an entry-level graphics card would've come across as absurd, but times are changing. Recently, GPU vendors have been lowering the PCI-Express lane count of their entry-mainstream GPUs in a bid to lower the pin-count and reduce traces on the PCB.



For the Radeon RX 6400, this means a PCI-Express 4.0 x4 bus interface—bandwidth comparable to PCIe 3.0 x8 or PCIe 2.0 x16. By itself, PCI-Express 4.0 x4 seems like plenty of bandwidth, 8 GB/s per direction, until you realize that you also need a PCIe Gen 4.0 capable processor and motherboard to use it in Gen 4 mode—that's Ryzen 3000 Matisse, Ryzen 5000 Vermeer, Core Rocket Lake, and Core Alder Lake. Entry-level parts from both brands are relegated to PCI-Express Gen 3.0.

If you're experiencing déjà vu, it's because we recently tested PCI-Express scaling of the RX 6500 XT, the slightly bigger sibling of the RX 6400, and drew some interesting conclusions, mainly that PCI-Express Gen 3 bites off a big chunk of performance. In my RX 6400 review, I suggested that the performance hit would be roughly similar, but several readers requested I actually test this theory, so here we are. We are hence testing the RX 6400 in PCI-Express Gen 3 mode, and evaluating its performance loss to Gen 4 to see just how much it affects the target audience of the RX 6400: those with entry-level platforms that have PCI-Express Gen 3.