Mostly it's a congestion issue rather than a bandwidth issue. But saying that, unless the separate WiFi is on another channel it will not help much. As for security, if they are cloud based it doesn't matter much, if at all, the reason to be totally local with your devices and hub. Non WiFi devices start off local then people connect to a cloud based Hub and at that point, once again doesn't matter. And totally local means no Google or Alexa support, but that is mostly just a fancy remote and not really Smart. The Smart is done in the hub, real automation.