Are you running the Z-Wave Poller? If so, try disabling it as it is known to put too much of a load on the Z-Wave network if too many devices are added to it. I used to poll some old devices and experienced laggy performance when I had too many switches being polled.
I personally grew tired of Z-Wave issues as I had a bunch of really old GE/Jasco Z-Wave (non-plus) switches and dimmers. Even on SmartThings these were temperamental and would require re-pairing frequently. I have since ripped them all out and replaced them with Lutron Caseta switches, dimmers, fan controllers, and pico remotes. I am amazed at how much better the Lutron equipment works in terms of reliability and performance. I do still have an Aeon HEM v1 Z-Wave device working happily, though.
For sensors, I prefer Zigbee contact, motion, and leak sensors. These all report status instantly with no lag whatsoever. I use Lowes Iris 3210-L outlets throughout the house to act as Zigbee repeaters (they are Z-wave repeaters as well!) I also only use Sengled Zigbee bulbs as they are not repeaters like most other Zigbee bulbs.
This configuration has been incredibly reliably and performance is great for me.