Although Processors are indeed important for gaming, It’s really not the only thing to considerer, 30% usage is by no means bad. For a game at least I’d say that’s decent, Could be a bit better but really not to bad. As for 8 GB being suggested for gaming pc’s. that is true, However it is unlikely that games will ever use that amount, Alpha games not included though.
I wouldn’t say the only solution is getting new processors to be honest, Some times you have to account for age of pc’s, (is it worth upgrading ect). Some situations can occur that something running on your pc is taking up resource’s without you knowing. Even viruses can do damage if they’re there and you don’t realize.
This game is in alpha, things like lag ect are going to happen. Every pc different, Some have better processors, Graphics cards, More ram ect.
Things you should probably see, Are other games lagging or just Stonehearth, Could it be bug related, Are things running using more resource’s than your pc can handle.
There’s a load of things to consider in lag situations, It could be something other than your CPU failing you, But it’s best to do some testing to really see. I’ve gotten major lag for having a loads of tabs open, So anything could be effecting it.
Hope that helped