It's worse than it was and the reason for that is not the declarativeNetRequest, but replacing background page with a service worker. You see, now extensions a

It's worse than it was and the reason for that is not the declarativeNetRequest, but replacing background page with a service worker. You see, now extensions are supposed to do background work in an ephemeral service worker. This service worker lifetime is very short (up to 5 minutes, then it's getting killed forcibly). So it's constantly getting killed and waked back up. Waking up includes doing some initialization which consumes additional cpu cycles. The situation will improve when Google implements the alternative to service worker (so-called "Offscreen documents"), but no one knows when exactly this will happen.