A critical resource is a resource that could block initial rendering of the page. The fewer of these resources, the less work for the browser, the CPU, and other resources.
Similarly, the critical path length is a function of the dependency graph between the critical resources and their bytesize: some resource downloads can only be initiated after a previous resource has been processed, and the larger the resource the more roundtrips it takes to download.
Finally, the fewer critical bytes the browser has to download, the faster it can process content and render it visible on the screen. To reduce the number of bytes, we can reduce the number of resources (eliminate them or make them non-critical) and ensure that we minimize the transfer size by compressing and optimizing each resource.