acusrola it's an interesting puzzle.
I found the link to the Daz3d GPU comparison chart:
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There wasn't an entry for the CPU 5900x. closest comparable was a ADM3700x, which is apparently about 0.55x your CPU's compute power.
The most recent benchmarks submitted for a GTX1080ti rendering the test scene result: 3.9 iterations per second
The most recent benchmarks submitted for a AMD3700X rendering the test scene result: 0.6 iterations per second.
Extrapolating, I'd guess your CPU alone would do about 1.1 iterations per second.
Neithier of these benchmarks are with latest Daz/Iray versions, which can sometimes make substantial differences in performance (*although recently most of the gains are in the RTX upgrades found on the NV2xxx and NV3xxx GPU series).
Regardless, it goes with my expecation that even a relatively old (but high end) GPU (the 1080ti) is approx 3-4x faster at Iray rendering than a CPU.
When you get into the RenderIterations/Second/Dollar (i.e. cost effectiveness) then the difference becomes even more pronounced (not withstanding the current GPU price gouging/scalpers and the fact the 1080 is no longer manufactured new).
The RenderIterations/Second/Dollar comparison is:
Ryzen 3700x = 06.906
NV GTX 1080ti = 20.560