Your implication is it’s limited to an interface, it isn’t. I’ve got significant delay in the interface, button reactions etc. I do have interface speed issues but not in isolation.
I’ve had several timeouts simply trying to connect to the front end.
The latest Core update has mediated some of them, but I’m still getting connection timeout.
Plus the <1% CPU is often > 1.5% and sometimes running at 25% which must mean systemically there’s more going on requiring additional processor usage.
Because you specifically asked I have 33 ZigBee devices on a SONOFF 3.0 USB running on a Pi4 4GB