How and where can I check that?Whats the upload speed of the connection of the NAS?
My upload speed is 5mbps but still the download off of external should be as low as 600kbps?you need to be connected to the network that the NAS is on and run a simple speed test from a computer or phone such as Speedtest.net. I'm almost willing to be that its the upload speed of the ISP thats limiting you. Most IPS's upload speed is much lower than the download speed. In todays times uploads are becoming more important which is why ISP's are starting to offer symmetrical upload and download speeds.