System and Topology

• TP-Link Deco WiFi 6 Mesh System(Deco X20), purchased December 13, 2020 - not sure what hardware version. 
• Operation Mode: Wi-Fi Router
• 3 units all wired and set for ethernet backhaul.  Two placed on ground floor, one on second floor of two story home with basement - 3000 sq. feet.
• Internet is Verizon Fios Gigabit, directly plugged into main Deco unit.  
• House is wired for gigabit speeds with 6 switches in the house.

Symptoms

System working fine until about 2 weeks ago when computers that are connected via ethernet have very slow response times from websites.  For example, Youtube videos take a long time to load/buffer or won't load at all.  Images on websites do not display.

Again.  Nothing really changed other than adding of more and more IOT devices (lightbulbs, outlets, etc.) - but keep in mind these are all WiFi enabled and they aren't having any issues (40-

Troubleshooting

1. Directly plugged a laptop (WiFi off) into the Deco unit; Fios -> TP-Link main unit -> Laptop.  Two other Deco units unplugged.  Speeds a little better, but same symptoms.

2. Disabled QOS

3. Disabled Parental Controls

4. Checked for updates

5. Optimized network

6. Checked all ethernet cables unplugged any that were not in use.  Ensure no loop in the network.

What else should I try?