Bulk file-transfer from a laptop running windows 8 will slow down the responsiveness of that laptop to other tasks as the CPU (rather than the network) is multi-tasking extensively.
As another alternative cause of your problem, take a look at this post
and see if 'buffer bloat' is a possible cause in your scenario. If it's the machine doing web-browsing which is suffering, and not the file transfer over the wireless network, then it won't be radio interference.
Hope that helps.