How am I meant to utilize Lighthouse to improve performance when I get such wildly inconsistent results each time I run it?

A customer wanted me to increase the performance of their site. Looking at it, it seemed pretty easy - lots of images that are too large, missing image height and width tags, etc.

Initial Lighthouse performance score was 20.

• I reduced the banner images from 5000px wide to 1200, score increased to 28.
• I reduced some thumbnail images from 650px to 400, score decreased to 20.
• I added lazy load on some images, score increased to 23.
• Removed some unused HTML, score decreased to 8.

Right now whenever I run the test I get anywhere between 16 and 30. Same with PageSpeed Insights.

How am I meant to use Lightouse/PageSpeed when they produce such inconsistent results?