There are a few things to keep in mind. First of all, the RFID tag read/write plus label printing is generally slower than a regular ZPL label printing. Secondly, the USB connection generally is slower than TCP connection. The sum of the two timeouts in SendAndWaitForResponse() API will be the max time to wait before the API returns. If we can shorten either one of them, we will shorten the total time to wait. One thing noticed in your post is that "null" is passed to the API call for the string terminator parameter. If we know the response string from the printer is ended with a specific string, we can pass that specific string as the terminator to the SendAndWaitForResponse() API call, so that the API call will return immediately after receiving that specific string. If we don't know the specific ending string, we can always use "\r" as the terminator because all response string will have it at the end. This way we can shorten the second timeout.
Another way to improve this is to use myConnection.write() and myConnection.read() directly, which bypasses the inherent delays and timeouts in SendAndWaitForResponse() API. Hope this helps.