Hello,
at first sight I saw nothing that could obviously explain such a difference between V3J4 and V3J10, if there is no other change in the context (which is my understanding of your case). The bitrate can still be configured at ST-Link UART level despite we do not commit on this value. We did nothing at firmware level to voluntarily decrease the performance ! The bitrate is one thing, and the amount of data we can process at this rate is another thing (the VCP bandwidth may be impacted by other USB interfaces for instance). The performance may also be impacted by the host context. The value of 15Mbps is the result of tests we did on our side, and for which we consider it is usable. But you should know that the VCP (based on UART) is not a protocol guarantying the data delivery, so some data loss is possible from time to time. If your application requires such data integrity, a handshake protocol between host and target should be implemented above. To conclude my fear is that there is a change which impacts indirectly the VCP performance, which will be difficult to understand, I'm sorry for that
