Can you help me improve my wifi connections on a C7100 Nighthwawk AC1900 WiFi Cable Modem Router?

[HomeTown]

Recently all my wireless devices (tablet, laptop, phones) are connecting at much lower speeds (90 Mbps) compared to my wired PC (550 Mbps.)  Is there anything I can do to get my wifi speeds back up?

None of my habits have changed- my router is in the same location, I have the same wireless devices, and I'm using them at the same distance from the router as before (my house is small so no device is ever extremely far from the router.)

[FURRYe38]

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

Set 20/40Mhz Coexistence to 40Mhz only.

Has a factory reset and setup from scratch been performed since last FW update? 

[HomeTown]

I'm using Auto on 2.4Ghz but it's using 11.   5Ghz is in channel 153. 

I do have about 7 Wifi neighbors nearby wth about 14 wifi devices in range.

I do not see where I can set Coexistence.

I did have Comcast do a modem reset.

[HomeTown]

I attached a picture from a WiFi Channel Scanner.