On a phone, on mobile data, visitors leave in seconds and never register as a bounce you could attribute. Speed work is worth doing because more of the people you already attract will stay.
In our experience, on a typical business site, roughly in this order.
Uncompressed, wrong format, and far larger than the space they display in. Usually the single biggest win and the easiest.
Chat widgets, analytics, heatmaps, consent tools. Each small, collectively dominant. There are usually tags for tools nobody uses any more.
Several weights of two families blocking text, and page builders that generate markup no amount of tuning saves.
Cheap shared hosting produces a slow first response that no front-end work can recover.
Speed is a ranking factor of modest weight. It will not lift a page that does not deserve to rank, and no amount of it compensates for a page that fails to answer the question someone searched.
Fix it because it costs you customers, and treat any ranking benefit as a bonus.
We reply with a view on whether we can help, and say so plainly when we cannot. No obligation.