Google has a separate tracking mechanism with regards to bounce/back to SERP rates which does not require GA at all. Time on site could theoretically be implied

Google has a separate tracking mechanism with regards to bounce/back to SERP rates which does not require GA at all. Time on site could theoretically be implied. It has also been (somewhat officially) stated that GA data is not used for ranking purposes, but make of that what you will. Since they also render pages themselves, they don't really need client page load metrics to evaluate rendering performance.