Probably flexing purposes
I’m pretty sure it’s how long it took to search for all of the results, still not sure why it’s there though. Maybe if someone is logging how fast google searches happen?
One answer not considered here: many services these days have a “budget” within which they’ll process your requests. If they take too long, it’ll mean another user isn’t getting served, and also that you could end up waiting a long time. So Google could just be cutting its search short and saying “let’s just give them what we have so we can better use our resources”.
It’s also known that the number of results Google gives you is actually an estimate. A vast majority of people will never past the first few results, and even less any other pages. If you’re at the scale Google is working at, every hundredth second shaved off is valuable.