I have also had slow wireless speeds when I used newer wireless keyboards and mouse. I have an old wireless keyboard and mouse and for some reason it lets it work just fine. The speed does drop off significantly the further you are away from the router.
You can use the Intel Driver update utility (link below) to update all of the adapters in Windows 8.1, but then you will need to do it again after you successfully upgrade it to Windows 10. You will have to have a flash drive handy 16gb to update the Bios and to upgrade to Windows 10.
I used the link below and followed every direction and I successfully upgraded it to Windows 10.
Please check Intel recommendations on how to upgrade the device to Microsoft® Windows 10.
Thanks very much!
I also try it later.
My answer, my trobuleshooting and below.
Device Manager -> Network adapter -> Wireless lan(Realtek) -> Property -> Advnced setting ->
BandWidth [5GHz + 2.4GHz -> 2GHz]
(sorry, viewing comment memo is lost, it is my memory.
I understood why it is fast to changes. It might radio wave interface.