No need to install Windows 10, I have a similar problem on W7. My desktop PC has wifi to router, my wife's is ethernet cable to router. After my PC goes to sleep it is slow to reconnect and speed is 0.5Mb instead of 11.2Mb. My wife's machine on ethernet works fine and our two iPads connect without problems.
This problem arose a couple of weeks ago after an MS update. I couldn't get it to connect at all until I went back using System Restore. All was well until the PC re-installed the updates, when the problem returned.
It seems to be widespread, some say update the drivers (I did) some say remove the VPN adaptor etc. I've given up and turned off the 'sleep' facility on the computer. Edited to add: Have tried scanning for other channels, there aren't any. An ethernet connection is probably the best solution.
Edited by Malwaremike (Mon 10-Aug-15 18:20:46)