Hmmm, it’s very strange that the devices don’t show up in the new app.
When migrating any existing devices should just show up in the new app (they did for me), there shouldn’t be anything special to do.
As far as I know, and in my experience, the list of devices should be exactly the same in both apps. The devices are already added to SmartThings, so they shouldn’t need to be re-added. It’s very strange that they’re not showing up for you.
They won’t appear under the device itself, since the new app doesn’t support “device tiles” which I used to show them under the alarm device before, but they should still appear as separate SmartThings devices under the main list of devices. I grouped all of mine under the an “Alarm System” ‘room’.
Regarding the slow response, the new app is slightly slower than the old app, but the response via the web interface (when refreshing the page) should be just as quick.
I wonder if the IP of your SmartThings hub has changed or something, and it’s failing to send the update to SmartThings.
Can you check that the IP address for SmartThings is correct if you go to the /settings page under the web interface for the alarm device ?
(e.g.: http://192.168.1.x/settings )