the Ryzen 3000 may be slight lower in max frequency but it does more work per frequency so its every thread will be faster than the 6700's thread, and there's more of it, so it will be much faster.
how much i don't know
when i was exporting 1000 photos the other day my 6700K took what felt like a couple of hours. i was watching the CPU load - it fluctuated from 50-100%. not sure why it had to drop down to 50% between the max loads but who knows.
either way, it will be faster.
whether it's worth the money only the OP can say. for me no, coz I don't do these exports often enough and when I do them I just leave it alone for a while. is it nice to have a ryzen 3900? yes! i'd also love to have an RTX 2080 Ti! but my 1080Ti combo can run games at ultra at 1440p at 144 fps so there is no need to go any better for now.