There is, effectively, no "virtualization" layer here.
There are some things that if needed can cause overhead... such as the bridge networking (really shouldn't be a bottleneck for majority of people), and the CoW filesystem... which docker won't be (or shouldn't be) running on top of since, for example, overlayfs on top of overlayfs is not supported.
There is also nothing stripped down about the daemon inside of the container.
Sure, I was speaking off the cuff based on my experience from a few years ago. Maybe I messed up and somehow had the DinD daemon not use a volume mount, and that's what caused it to build images slowly?
There is, effectively, no "virtualization" layer here. There are some things that if needed can cause overhead... such as the bridge networking (really shouldn't be a bottleneck for majority of people), and the CoW filesystem... which docker won't be (or shouldn't be) running on top of since, for example, overlayfs on top of overlayfs is not supported.
There is also nothing stripped down about the daemon inside of the container.