Recently, my home LAN experienced a sharp drop in the SMB throughput. I then checked it with an HTTP transfer, and it was also very low (~800 Kilo-Bytes-per-second, or less than 10 Mega-bits-per-second). However, we were able to resolve this by restarting our switch. Now our speeds are back to their normal high.

So this solution may work for you too in case something like that happens.