I don't consider being paid to give a talk a bonus, let along a performance-related one. It's a fee. By "performance-related bonus" I mean specifically rewards that come (at least nominally) as a result of meeting performance targets, or of exceeding expectations.

I can well believe that the tethering system you describe exists, but it seems to me that it isn't performance-related pay but rather deferred pay, and that it's dependent not on doing a good job but simply on not staying with the company.