Upload performance is less impressive than download

I deployed one chat server and opened one chat client in different countries. I used this single client to profile download and upload performance. The download speed could reach about 15Mbps, which is OK. But the upload speed never reached 1Mbps. The server continued processing messages for some time after client finished sending all the messages. No message was lost on the server side. It seems there is some throttling for upload.

If there is throttling, where can I find out the code for throttling? If there is not, how can I improve the upload speed?

Thank you.