Debian's kernel by default comes with and loads in the DRM/KMS i915 kernel module.The Debian and FreeBSD installations are without GUI.
With FreeBSD you should do that too or it will be using scfb or vesa which has very basic power management. This is important, even if you aren't planning on running Xorg. It affects the framebuffer terminal too.
I.e in theory, Windows has the best power management for consumer amd64 hardware but if you also run it with the "Basic Video Adapter", it is pretty terrible too.
Otherwise, I might suspect it is because FreeBSD has fairly early Intel p-state support. It might be disabled by default due to this bug.