First off Ver. 12.0.104.000 this problem didn't exist.
I'm using Win7 Pro 64 IE Ver. 11.0.9699.16521
My speed tests and downloads slowed down by about 18Mbps on a 100MBps connection. It runs about 105Mbps and slowed to about 87Mbps.
Through process of elimination I figured out what was causing it.
1. Go to Antivirus/Anti-Spyware Advanced Settings
2. Premium Protections
3. The Check Box Enable Web Monitoring - Uncheck it
Unchecking this restored all my original speed of 105Mbps on speed tests and downloads.
I don't know the specifics of how the Web Monitoring is working within the program itself as I don't have access to the source code.
Hopefully you can figure out why this is occurring, until then I'm leaving the Web Monitoring box Unchecked.