It's super powerful, completely configurable and extremely effective at correcting any issues that may come up from time-to-time that effect speed or performance. It's created and maintained by just one guy and has been since 2003. The best thing is that it's completely free.
What needs improvement
The UI is very 2003, so some mightn't be so enamoured with its appearance. But I personally find it has a certain charm. It's no-frills design gives me faith that the time's been invested primarily in the product.
Others are often heavy-handed with MacOS. They're resource-intensive, intrusive and they don't actually do much work. Onyx is lightweight, runs only when you want it to and actually does more than any paid product.
MacPaw