@matink 

Everyones config is different so what works for me may not work for you.

Some general rules to consider:

Location is key for WIFI as placement and even location, height, and direction the router is pointed makes a difference.

Allways use Ethernet when possible. Use a switch if needed.

Use Cat 6 or better cables. 

Use Ethernet backhaul for instead of WIFI for MESH, APs, and extenders. 

Keep number of APs. extenders, and MESH units low as possible.

If using WIFI as backhaul, locate the Extender, AP, or MESH unit in a good signal location to the router. The better signal it gets, the better signal it can pass on. 

Keep the software up to date. 

Upgrade your equipment. Example old PCs may be using 100 for an Ethernet connection. 

Old equipment using old WIFI standards can drag down the WIFI network. 

Always split the WIFI networks to control which devices are on which network. 

2.5 GHz is better for distance and 5 GHz is better for speed. Limit what is on your 5 GHz if you are concerned about speed for selected devices. 

If in an area with a lot of WIFI from others, you may need to use selected channels and other parameters, 

Using the router's USB for backups takes clock cycles from other router tasks. 

Some devices may hog the network such as if you have a lot of IP cameras connected to the router instead of an NVR. 

I am sure there may be other things others can add.