Just to add to others who are posting some things that have helped;
I noticed I had poor 2.4 GHZ range performance on my wireless, but great 5 GHZ. Smartthings requires an ethernet connection to your hub, and I have a Apple Extreme which is pretty powerful. There was a lot of contention with them siting close together. I regained better control of my Zigbee devices, and improved my general 2.4 GHZ performance on other things around the house by moving the hub a significant distance.
I have seen recommendations to use a long ethernet cord and place the hub 15-20 ft away (or other side of room from wireless device). In my case, I used an Apple Base Station to extend the network, placed in the kitchen (40 ft away) and attached smartthings to the network port there. Everything is much better, about an hour after the move as I figure zwave and zigbee were rediscovering themselves.
Just a point in case it helps any others as things got better. I also removed Harmony connect after watching how danged chatty that was every single second…never got much value out of the Harmony connect.
As an amusing side note, if you use parental filtering devices like Circle, make sure you exclude your SmartThings hub so its not blocking internet to it after an hour of usage. LOL. (for parents: https://meetcircle.com/)