Honestly, the biggest gains I have seen come from places most people overlook when they are fixated on budget.
The first thing I always audit is audience targeting, not because it is the most exciting lever, but because wasted impressions are silent budget killers. A campaign showing ads to people who were never going to convert will always have poor ROAS regardless of how good the creative is. Tightening the audience, excluding irrelevant segments, and building lookalikes from actual buyers rather than general site visitors makes a real difference.
Creative testing is where I have personally seen the most dramatic shifts. The same audience, the same budget, a different creative angle and suddenly your cost per conversion drops by half. The mistake most people make is testing too many variables at once or pulling the plug too early. You need enough data before you call a winner, and you need to retire fatigued creatives before they quietly drain performance.
Landing pages are probably the most neglected part of the equation. If your ad sets an expectation and your landing page does not immediately deliver on it, people leave. It sounds obvious but I have audited campaigns where fixing just the message match between ad and page moved conversion rates noticeably without touching anything else.
Bidding strategy is another one. Once you have solid conversion data in the account, shifting to a target ROAS or target CPA bid strategy tends to outperform manual bidding. The algorithm optimises things at a speed and scale you cannot match manually. That said, this only works when your conversion tracking is accurate. Messy data produces messy results.
The last thing worth mentioning is attribution. A campaign that looks weak on last-click attribution is sometimes driving a lot of assisted conversions that you are not crediting it for. Understanding the actual customer journey helps you make smarter decisions about where to keep budget versus where to cut.
None of these work as standalone fixes. But in my experience, tightening the audience and improving landing page relevance tend to show results the fastest.