I am not sure how the containers are handled with the zfs storage driver, so I can only speak for the Overlay2 storage driver.
In Overlay2, the container filesystem is a virtual filesystem that is composed of all images layers and a copy-on-write-layer that allows write operations. Doesn’t it kind of depend on the application how much data is accessed from the container’s filesystem.
I don’t even remember If I ever ran the docker root dir from an hdd (/var/lib/docker). I always used a ssd, first with SATA interface and since a couple of years with NVME interface.
Update: it seems zfs is quite similar when it commes on how the container’s filesystem is composed (see: Use the ZFS storage driver | Docker Documentation for details), but offers additional features compared to Overlay2