I had this same issue; many have, where single threaded downloads crawl; i was getting 300k - 2 megs max. after i did the following I got 35+ megabytes/s. You may have to disable hidden and visible protocols. This worked for me on two separate occasions; interestingly, speeds were still fast after disabling then RE enabling them; (unlikely but maybe I did not restart my lan adapter) or maybe its just a windows bug. be sure to do a windows restore prior to just to be safe. you may disable one at a time and see which one is the culprit. if need be you may isolate and disable only those which you know you are non essential to your current setup; for example, to ensure lan networking; ms_implat has to do with network card bonding/teaming. Remote Direct Memory Access (RDMA) works on enterprise network cards and is not commonly used by workstations.
in short, download nvspbind, extract & at a powershell prompt run the following:
.\nvspbind.exe /d * ms_ndiswan
.\nvspbind.exe /d * ms_server
.\nvspbind.exe /d * ms_rdma_ndk
.\nvspbind.exe /d * ms_implat
.\nvspbind.exe /d * ms_netbios
.\nvspbind.exe /d * ms_lldp
.\nvspbind.exe /d * ms_tcpip6
.\nvspbind.exe /d * ms_pppoe
.\nvspbind.exe /d * ms_lltdio
.\nvspbind.exe /d * ms_server
.\nvspbind.exe /d * ms_netbt
.\nvspbind.exe /d * ms_msclient
.\nvspbind.exe /d * ms_pacer
/d = disable, * = ALL windows adapters, unused/non essential ms_protocols;
just run "nvspbind.exe" to see more details.