edwin_harris Hi there,
Interested to see if we can pin down what's happening here. I've not got a Ubuntu machine to test on myself, but I do have an old desktop running a Core2 6300 with Win 10 installed (for which I had to up the RAM from the original 2GB to 8GB...).
I'll install Firefox to have a look at how that behaves later, but for now I've had a play around in Chrome and I'm not seeing the same as you. I get CPU load spikes whenever I load new content in the community, but it rapidly subsides again once the content is loaded - exactly the same behaviour as I see on other sites.
So I think we'll need to look further into what might be the cause for you. Are you, for example, running any browser extensions (my Chrome is stock)? Are there any particular pages on the community that are causing it, or is it like that regardless of where you are in the site? Is your version of Firefox fully updated?