Ok so I have been fighting this issue for 3 days with no luck! I'll provide as much information as I can and hopefully someone may know a fix or at least some more steps I can take in fixing this issue.
This is a home user, Sky Fiber using Sky's router at just over 20 Mbps download. His LAN consists of 1 Windows 7 laptop, 1 Windows 8.1 PC, 2 iPhones.
Windows 7 laptop - speedtest.net shows consitant download speeds of 18-20Mbps
Iphones - speedtest.net shows consitant download speeds of 18-20Mbps
Windows 8.1 PC - speedtest.net shows 3-5Mbps download although at random times during the day can reach 18-20Mbps (not often)
The windows 8.1 PC was previously Windows 7 when this issue started. From the begining on Windows 7 - I upgraded network adapter drivers to the latest version, updated bios & chipset drivers to latest versions with no change. I performed virus/malware scans and found the PC to have a fair few viruses and malware. I did numerous removals with no change. Safe mode with networking worked fine! At this point I decided it may be a very nasty Virus/Trojan/Rootkit of somekind that is evading the anti-virus/malware programs. He had Windows 8.1 and was looking to upgrade his machine anyway so we though it would be a good time to format the drive and do a fresh Windows 8.1 install.
Windows 8.1 - installed the latest drivers, chipset, networking adapter, graphics card etc. and still no change. Speed still around 3-5Mbps ocassionaly hitting the expected speed of 18-20Mbps but not very often. Safe mode with networking still working fine. At this point I'm thinking a driver issue.
Tried with a wired connection no difference, tried taking the wireless adapter out of the machine, unistalling the drivers and then testing wired connection, same issue. Tried disabling IPv6, same issue. Tried turning off the option - allow the computer to turn off this device, same issue. Tried changing the local computer policy that resevers bandwidth - forgot the actual name - however no change. Tried with Windows firewall off and A/V off no change. All the while it is working in safe mode with networking.
Motherboard - Gigabyte GA-F2A85XM-HD3
Wireless Adapter - Edimax EW-7612Pin V2 (Has a Realtek chip)
Wireless Adapter Driver - Realtek RTL8192CE Wireless Lan 802.11n PCI-E NIC
Driver Date: 11/22/2013
Driver Version: 2012.4.1022.2013
So any help with trying to resolve the issue would be much appreciated, a question in my mind is that if the safe mode drivers work fine, could I use these drivers in normal mode? Assuming the drivers are at fault.