Z-wave Intermittent Delay - particularly with Fibaro Dimmer 2 and Single Switch Devices

I recently purchased the Samsung SmartThings Hub, and have been testing light switches (in particular the Fibaro Dimmer 2 and Single Switch). I have custom glass touch sensors, and require wired switch inputs to interface my touch switches to the actual dimmer/relay. I only found Fibaro and Aeotec (both Z-Wave) and have started testing with Fibaro. Ultimately, I need to install about 150 dimmers/relays in a large home, but am starting with a small number of devices for testing.

My chief concern is whether Z-Wave is consistent and reliable enough to avoid long delays (more than a few seconds) and intermittent operation.

The ST Hub interface only shows the Single Switch or Double Switch (but not the Dimmer 2) if adding manually. But I was able to add the Fibaro Dimmer 2 automatically by pressing its “B” button 3 times.
Both are working fine, with the exception of intermittent delays.

About a week ago, I installed 3 switches. One is about 20 feet from the ST Hub (near the bottom of stairs). A second is at the top of stairs (about 20-25 feet away). And the third is in a bedroom around the corner about 30-40 feet away). All 3 seem to work fine (minimal delay in communicating status and operating remotely).

However, I experience intermittent failures/delays (no communication, or 10+ seconds delay) when testing. I havent yet detected a precise pattern, but the delays tend to occur when I havent tested the switches for a few hours or overnight.

My problem is that I dont know what I can do to troubleshoot the problems. I dont see any parameter settings on the Fibaro devices, so I suspect this is a generic Z-Wave issue (though I havent yet tried other Z-Wave devices).

I tried the Z-Wave Repair, which occasionally fixes the problem temporarily (but not always). I also tried removing power from the ST Hub, which seems to help (which is odd since it doesnt really reboot given the backup battery).

Does anyone have suggestions for troubleshooting Z-Wave delays? I want to test the problem with a handful of Fibaro switches before investing in 150 devices.