A bloated index slows your good pages because bloat is a tax on the whole site, not a problem confined to the weak pages themselves. The low-value pages dilute the site’s overall quality signal, waste crawl budget on URLs that do not deserve it, and scatter internal link equity across pages that should never receive it, so your strong pages end up competing from a weaker site-wide base than their own quality would suggest. The mistake is treating each page as if it stood alone, when the search engine forms a view of the whole property.
Start with the quality signal. The search engine reads the body of your indexed pages to judge what kind of site this is, and a large share of thin, duplicate, or templated pages lowers that overall read. Your genuinely good page inherits the site’s reputation, so when the typical page around it is weak, the good page is judged against a discounted baseline. It has to work harder to rank than the same page would on a clean site.
Then there is crawl. The search engine spends finite attention crawling and re-crawling your URLs, and every low-value page in the index is a place that attention can be wasted. When crawl is consumed on bloat, your good pages get discovered later and re-crawled less often, so updates and new value take longer to register. Internal equity behaves the same way: links and authority that flow through the site get spread across hundreds of pages that contribute nothing, thinning what reaches the pages that matter.
So if your strong pages are underperforming their quality, look past the pages themselves to the company they keep. Cut the bloat to lift the site-wide base they rank from. Removing or noindexing the low-value pages concentrates crawl, sharpens the quality signal, and stops scattering equity, which is how trimming pages you do not care about ends up helping the ones you do.