Slow wi-fi, highly variable wired connectivity

Hi Everyone:

  I have had a difficult time diagnosing and resolving issues with slow wi-fi speeds. My network consists of the following:

• Netgear C6300 Cable Modem/Router running IPv4
• IBM PC running Windows 7 (about two years old, 16 GB RAM, 128 GB SSD (primary) and 1 TB platter HDD, gigabit nic), hard-wired to network
• 25 Gbps Comcast Internet connection
• Wi-fi with two password protected networks (auto channel on low end, 153 on high end), guest accounts disabled
• Common connections to wi-fi networks for two iPhones
• BluRay player connected to wi-fi for Netflix
• Occasional connection with Denon receiver
• Brother multi-function printer, hard-wired by wi-fi capable
• ADT security camera, wi-fi to receiver, wired to router
• Solar City wi-fi to receiver for monitoring solar array, wired to router

My typical speeds are as follows:

Comcast as measured on desktop pc hard-wired to router: 15-88 Gbps download, 5-15 Gbps upload. System sped-up considerably after a call to Comcast and a clearing of network traffic that had built-up.

Measurement Lab wi-fi speed test as tested from iPhone on wi-fi connected to the higher frequency network: 1 Mbps download, 5-7 Mbps upload.

Comcast assures me that the firmware on the router is up-to-date. But my suspicion is no longer that the problem lies with the ISP. I also do not believe that the router is the problem for the wired network. The problem seems to be confined to the wi-fi networks.

What I have tried:

• Detecting other networks and switching channels to lesser-used frequencies. There are about five other networks in my neighborhood.
• Lowering the MTU.
• Switching to IPv6 (which yielded a temporary, minor improvement).
• Performing a factory reset (multiple times).
• Reloading the firmware as obtained from the Netgear site.
• Analyzing typical network traffic (not seeing particularly heavy loads under normal circumstances).
• Locking-down access to the modem/router.
• Significantly increasing the complexity of wi-fi and admin passwords (eye toward a hacked router siphoning bandwidth).
• Moving the router to different positions and observing the speeds from different parts of the home (ensuring wi-fi appliance is not too close to the router).
• The slow connectivity is the same at night as it is during the day - ruling out that the solar inverter may be causing EMI.
• All computers and accessories that are hard-wired to the router have been powered through a surge protector for more than a year - we bought the router about six months ago. Cable line also goes through the surge protector. 

I'm handy with computers, but I would not describe myself as an expert. Which is why I am bringing my concerns to the community. Hopefully there is an answer for me here. Thank you in advance. I appreciate it greatly.