According toTechspot Chromebooks use a lightweight, limited OS that's far less demanding on the spec. While manufactures have used this to churn out slim Chromebooks, Neverware have hit on the idea of running the software through older machines.
Neverware's CloudReady software, free for individuals creates your own Chromebook.
It is not a real Chromebook of course. Google owns the trademark for that name and insists on all sorts of specs for it. The software is a variation of Chromium, the open source version of Chrome.
However since your old laptop or desktop is probably still faster in terms of raw processing power than most new Chromebooks it will turn the computer a usable system absolutely free.
If you have an old laptop lying around it might be worth giving it a second life. You can download the software here