My CPU is double-core 2.5 GHz.
But most times I limit them to 1 GHz.
You can do this with packages like cpudyn in Debian.
DVD-Rom... I plug the cables out coming from the Motherboard. DVD-Rom makes unnecessary noise and consumes unnecessary electricity.
I also don't need it.
There is also Parted Magic, it's a very small Linux which you can boot from USB-Flash and after that it is loaded into RAM of PC. So you can plug-out the USB-Flash again. This way your HDD does not consume much energy, because nothing is read or written onto the HDD.
I use this mostly as a rescue-system, because you can mount every HDD (Windows/Linux format) into the operating system which runs in RAM and if you chroot the HDD you can also open a shell to the Operating system which is installed on that HDD. So, for example you can update or remove packages through the Shell, which caused the original operating system on the HDD to crash.