I accidentally crippled my 4x RTX 3090 LLM rig with a hidden PCIe 2.0 x4 slot and fixing it doubled Mistral 128B performance

In three linesA user discovered one RTX 3090 was connected to a hidden PCIe 2.0 x4 slot on a Gigabyte X399 board, crippling performance to 11 tok/s on Mistral 128B. After repositioning GPUs and proper tensor-split configuration, throughput doubled to 24.7 tok/s. Warning for multi-GPU builds on older HEDT boards.

Summary generated by Claude — human-verified