It's also good for performance. The blocking can happen immediately in the browser/network process, instead of waiting for the extension code to run in its own process and tell the network service what to do.
In principle yes. But in practice, we are talking about nanoseconds; and I would very much like to see benchmarks/measurements showing anything that can be perceived by users. Also, this blocking cost is still orders of magnitude lower than network latency and blocking requests (even with a slow adblocker) will result in a noticeable performance boost while browsing the web.
It certainly does not take merely nanoseconds to wait for JS to run in another process.
This comparison is only apples to apples if exact same content can be filtered. If you lose some filtering due to the added restrictions on blockers, the page may load more resources (in particular, javascript), easily negating any CPU performance benefit.