Hi again ecoalex,
I was discussing your case with an engineer and he checked on our side to make sure there wasn't something happening: no alarms, just some bad weather for your area yesterday. However, after digging further into the devices experiencing the high delay, I realize these Dell models, the Optiflex 790, are 10 years old. Considering that both of these Dells are showing high delays, I'm inclined to think that their NICs (network interface card) are the root cause, especially if only the Dells are the only devices experiencing these disconnects.
I'd look into replacing the NICs or upgrading your PCs.
As I mentioned before, the engineer saw nothing on our side indicating any issues. The device names I'm seeing with the high delay notification are labeled "Optiflex 790", are they not the Dell Optiflex 790 model? While the PC may be newly acquired, the model is still old, and I'm concerned with the NIC in those PCs. If hesitant to replace the whole PC, try just updating the NIC to see if that makes a difference.
You can also try changing the wifi channel; perhaps put each PC on different channels, just be sure to pick from either channel 1, 6, or 11.
You should be able to pick channels from the advanced settings in the wifi settings menu. For reference this shows how to access your wifi settings.