I had the same issue when setting my OpenSprinkler to DHCP and assigning a static IP to it within my router while connected via a wired Ethernet conenction.
I found using WireShark that the OpenSprinkler kept sending DHCP Discover and DHCP Request messages 5 to 6 times a second.
This would cause the OpenSprinkler to be slow to respond to button presses and the screen would flicker.
I am using a DIR-882-US AC2600 EXO MU-MIMO Wi-Fi Router on the latest firmware.
I resolved the slowness by setting a static IP address within the OpenSprinkler web application but now I’m seeing frequent instances where the OpenSprinkler stops responding to web requests and requires a reboot. Oddly enough it continues to respond to pings, but just not http requests.