That's true. But the size difference between alpine and debian:*-slim (for example) is on the order of < 20mb. Datacenter hardware can download this in milliseconds. There may be some rare scenarios where this is a meaningful delay, but usually provisioning time is bottlenecked somewhere else. Heck, it probably takes longer to download the VM image (and if you're optimizing for provisioning speed, you shouldn't be downloading Docker images on boot anyway - you should be baking VM images ahead of time.)