Fixes high CPU usage while Brave ads is enabled - 1.30.x

[tmancey]

Uplift of #10372
Resolves brave/brave-browser#18523

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[kjozwiak]

Closing as per the discussion between @jsecretan & @tmancey. We'll try getting this one into 1.31.x if possible but will discuss via the b-b planning/triage call today.