Performance issue

Greetings,

I have recently experienced noticeable lag and slowdown in my wifi, and for the first time noticed a message stating Performance Recommendation pop up next to my home wifi network (viewed on my cell phone).  I have a N600/WNDR3700 router, along with a NETGEAR DOCSIS 3.0 High Speed Cable Modem Certified for Comcast.  I do not see any updates for my router, and wonder if it's simply time to replace it.  Regarding the dual-band signaling feature (2.4 GHz and 5 GHz), I only use the former as the 5ghz option always has a weaker signal.  My firmware is V1.0.7.98NA, and My OS is Win7 32Bit,

I have a NETGEAR N300 Wi-Fi Range Extender and use that upstairs in my home, but this hasn't caused a problem in the past. 

My Comcast Account Internet plan is Performance Pro, calling for the following speeds:

Download Speed: 50Mbps*
Upload Speed: 5Mbps

Here is a recent Comcast Speed Test: Download speed of 27 Mbps, Upload of 5Mbps

Any thoughts about what could be going on?   

Thanks