Mining rig gives little speed increase

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There are two bottlenecks when running a dictonary attack with this setup, because candidates are processed by the CPU first and then distributed to the GPUs:
1. A presumably shitty CPU
2. GPUs, which are probably connected with USB Risers

This is exactly the reason why mining rigs are not suitable for password cracking. Have a read here, if you want to know more about hardware recommendations for password cracking: https://passwordvillage.org/hardware.html#rig_setup