Perhaps the Windows 8 PC has 3rd party firewall software (e.g. Norton) that is monitoring your network traffic. This will slow down Internet communication at the software level. Remove Norton or other 3rd party firewall to test.
Perhaps the Windows 8 PC has spyware. Run a spyware scan (MSE, MalwareBytes, or SuperAntiSpyware) or have someone do clean install from your recovery partition.
Rant (no need to read this part): In general Windows 8 is a bad operating system. Microsoft traditionally releases new versions of Windows with inefficient and buggy code. With the exception of Windows 7, every newly released Windows is almost unusable compared to the previous version. However, they slowly fix the OS by sending out service packs.
Example #1:
1) Windows Vista is released (very buggy, tons of new useless features to slow you down)
2) Windows XP (previous OS is all fixed up if you get the latest service pack)
Example #2:
1) Windows 8 is released (very buggy, tons of new useless features to slow you down)
2) Windows 7 (previous OS is all fixed up if you get the latest service pack)
Example #3:
1) Windows 9 will eventually be released (it will be very buggy, with tons of new useless features to slow you down)
2) At this point Windows 8 will be the previous OS, and will finally be all fixed up if you get the latest service pack)
Lesson: Try my suggestions above, but keep in mind, you will always have trouble using the current version of Windows. Instead of hiring lots of good code-testers and normal-user testers, Microsoft simply releases buggy products that everyone hates and lets the public find the bugs and scream about all the new annoyances (such as having to find a secret menu, then find the settings section, the find power section, simply to turn off your computer in Windows 8). Therefore, the previous version is always better.