There is some argument about how bounce rate is measured. Google has even changed how it looks at bounce rates by adjusting how it views a visit through the years. In short, Google says that the bounce rate is the rate where a user has left your site from the entrance page.
This page seems to explain some of what Google sees as a visitor: https://support.google.com/analytics/answer/2731565?hl=en
Google relies upon cookies to track usage. Here is a link for that: https://developers.google.com/analytics/devguides/collection/analyticsjs/cookie-usage
Unfortunately, Google does not do a very good job of separating bots from users.
It is frustrating because Google has not been able to distinguish a user from a bot very well thus driving up the bounce rate in GA rather severely. One would have to assume that this translates into other metrics as well and can effect site performance.