What happens if you configure as stock and use the stock setup of the OS? It should return to it's prior good performance. All those drives mean that there can be more overhead as Windows indexes and more. As this is quite a dated OS I'm shortchanging everyone on the old discussions about Windows Indexing. It's been a decade and there's plenty to read now about such things.

No matter which CPU or such you install all will result in more CPU usage as we play movies and browse the internet. As presented there is no cure for that issue.
Bob