A Ryzen 9800X3D is about 40% faster in single-core tests and the same speed to slightly faster at multi-core tasks, as compared to the M2 Ultra in the Mac Pro. In addition, the Ryzen computer would presumably be modular and allow for the user to choose their preferred configuration of memory, storage, GPU, etc, with options far exceeding those offered by Apple in its limited and non-user-upgradable machine. In addition, configuring the Ryzen machine with comparable specs to the base model Mac Pro (64GB of ram, 1TB of storage, and a low-end to midrange discrete GPU) would put you at a total system cost of something like 20-25% of the $6999 that the Mac Pro cost, even with today's inflated memory prices.

I'm not sure if this is what the parent meant by "a real modern PC," but it would certainly be 1) faster and 2) vastly cheaper than the Mac. So at minimum, your assertion that it'd be slower is wrong.

Depending on your configuration, you could likely also match the overall power consumption of the Mac as well, though yes, it is easily possible to exceed it. But the most likely way you'd exceed it is with a high-end GPU, which would vastly outperform the (fixed, non-upgradeable) GPU in the Mac.