Why is my rendering so slow?

Good news! I have a shiny new Ryzen 7950x to replace my aging i7-3770k!
Bad news. It hasn't really improved rendering times.
While rendering video, CPU usage seems to hover around 30% (with the fans barely ticking over). RAM usage is low, I/O is at maybe 80MB/s on a drive which can do way more than that, I'm reading from one drive and writing to another... I think I'm doing everything right?
What could be causing the performance issues?
(Lite user, Linux.)
Bad news. It hasn't really improved rendering times.
While rendering video, CPU usage seems to hover around 30% (with the fans barely ticking over). RAM usage is low, I/O is at maybe 80MB/s on a drive which can do way more than that, I'm reading from one drive and writing to another... I think I'm doing everything right?
What could be causing the performance issues?
(Lite user, Linux.)