Most pages are heavy because of their images. The fix is rarely a single magic setting: it is choosing the right format, resizing to the size the image is actually displayed at, and then compressing. Do those three in order and a 4 MB photo routinely becomes a 180 KB one that looks the same on screen.
Resize before you compress
A photo from a modern phone is often 4000 pixels wide. If it appears in a 800 pixel column, more than three quarters of that data is discarded by the browser after it has already been downloaded. Resizing to roughly twice the displayed width covers high-density screens and is the single biggest saving available. Compression applied to an oversized image is effort spent on pixels nobody sees.
Pick the format for the content, not out of habit
JPG suits photographs, where gradual colour changes hide compression artefacts well. PNG is lossless and suits screenshots, diagrams, logos and anything with sharp edges, flat colour or transparency, but it is a poor choice for photographs because it cannot discard detail the eye would never miss. WebP handles both cases and usually produces files 25% to 35% smaller than JPG at the same visual quality, with support in every current browser.
The common mistake is saving a photograph as PNG. Converting that file to WebP or JPG often cuts the size by 80% or more with no visible difference.
What the quality slider actually does
Lossy compression throws away information the eye is least likely to notice, mostly fine colour detail. The quality number decides how aggressive that is. In practice, 90% is near-indistinguishable and still large, 75% is the sweet spot for most web images, 60% is usually fine for background and thumbnail images, and below 50% artefacts start showing in smooth areas such as skies and skin.
Quality numbers are not comparable across tools or formats. WebP at 75% and JPG at 75% are different amounts of compression. Judge by looking, not by the number.
How to check you have not gone too far
Compare at the size the image will be displayed, not zoomed in. Look at the areas that suffer first: skies, shadows, out-of-focus backgrounds, and any sharp edge between two flat colours. If you cannot see banding or blockiness there at normal size, the setting is fine.
Compress once, from the original
Every lossy save discards more information, and it never comes back. Compressing an already-compressed file compounds the damage. Keep the original somewhere and always export from it, rather than re-saving the export.
A workable default
Resize to about 1600 pixels wide, convert to WebP, set quality to 75%, and check the result at display size. That combination handles most web images without further thought. You can do all three in the image compressor, which shows the exact saving for each file so you can see whether a lower setting is worth it.