High CPU usage compared to x11grab

x11grab with `-c:v libx264 -crf 0 -preset ultrafast -threads 0` has around 40-50% (single core) CPU Usage.
wf-recorder with `libx264rgb` has around 120-140% CPU Usage.

Can I make wf-recorder behave more like x11grab in terms of CPU usage by some command line option? If not, how comes it needs > 2x the CPU usage?