I don’t know how long you’ve been using smart vents, but my experience is very different from yours (and most of my contributors). I’ve been using the Keen Vents since the Keen beta testing (more than 2 years ago), and I haven’t had any ST performance issues. I have more than 100 connected devices, and 16 smart vents with no performance issues whatsoever.
You may want to contact ST support as I suspect that you have some zigbee mesh issues.
Some of my contributors have also used the Keen app for a while, and they came back to my zoned heating/cooling solutions as they have way more features and configuration options.
Here are some recent testimonials:
Regards.