Have you checked to verify if this is a network issue on your server? For instance, does it happen from multiple internet connections, or does it happen for any other servers that you operate?
Upload or download speed are mainly depends upon your local internet connection. If you are sure that it is not too slow then check your server's network speed by ethtool or miitool and confirm network card's speed if it is running with half duplex mode or full duplex mode.
Have you also checked if there was high load on server while you download sql backup via phpmyadmin. Sometimes it could slower the server speed.
Another option is to try changing server resolver and check if it make any improvement in server speed.