The 2009 Mac Pro has two PCIe x 16 slots. The reason I suspect was due to the fact the the bus speed of the Thunderbolt 3 connection to the eGPU enclose is only 1/4 of a full PCIe x16 bus. Recently, I noticed that although the eGPUs allowed for my MacBook Pro to have access to a full-fledged GPU, the performance was nowhere near that of a GPU that is housed on a regular PCIe slot. Luckily for me, I have two eGPU enclosures both housing AMD RX 580 GPUs. Frankly, these are fine for my purpose but note that the standard SATA connections on the motherboard of the Mac Pro would make getting a larger SSD an easy upgrade. The system came with a 160 GB SSD drive and a SATA 640 GB HDD.
