I've just realized my Raspberry PI is running on a slow SD card. It maxes out writes at 4.8MB/s.
The RPI SD card slot should be able to do 10x that (~40MB/s) and if I upgrade to RPI5 I could get up to (~85MB/s).
With RPI5 I'd also have the option of connecting a NVMe SSD and get 450MB/s!