We are often very slow when uploading using transporter, and we often get stuck during upload and cannot continue uploading.
The solution is to use a third-party online upload tool. Here I recommend you to use the "banana cloud editor" tool to upload. You can search for this tool.
As for why it is slow? The reason is simple, because Apple's server is in the United States, and the link between our broadband and the US server is extremely unstable.
Therefore, we can use some third-party tools to upload ipa files on a stable network. The networks of these third-party tools generally have foreign servers. This avoids direct connection with Apple servers and reduces upload instability.