I also having issue with my Lenovo Y510P. Just got off the phone with Lenovo tech support and they are saying the hardware is working as designed even after he agreed that he is seeing the wifi speed throughtput fluctuates. The wifi speed would start up at 4-5MB/s and then drop down to 1MB/s and climb back up again. The fluctuation is unpredictable. The hardware guy told me to call Lenovo software support, not sure what that is going to do.
I did the exact same test with an Acer netbook 4 years older than this Y510P and it have no problem with the wifi speed being constant.
Using driver 15.12.0.6, this is the latest driver that will install using the windows driver update option.
I tried Breen solution above and there was no help.