You can tell if a machine is the server computer if it has a white dot on it on the Synergy config app. Generally, mouse lag happens on macOS when the macOS is the client computer and not the server computer.
This wifi lag could affect your Synergy experience, causing mouse lag where the mouse cursor movement appears delayed or choppy. This is also known as a "ping spike" (named after the ‘ping’ tool). On macOS, the wifi adapter can sometimes lag randomly. Other than slow wifi, the only possible cause could be a problem with the CPU or memory problems that can cause the mouse cursor to lag. There are currently no known issues in Synergy for the Windows and Linux operating systems that could cause mouse lag. If your computers don't have ethernet ports, you may need to purchase a USB ethernet adapter.
If your ethernet network is in another room, or you don't have one, then you could set up an ethernet network. The best solution is to use an ethernet cable. You can check if this is the issue by doing a ping test. The most common cause for a mouse cursor to lag is usually a bad wifi connection.