Synergy lags extremely hard
On older versions of Synergy LAN is much better than WLAN for Synergy. If Synergy has to connect via WiFi at any point, you will certainly get lag problems. I was just having this problem and I solved it by connecting an ethernet cable to the two computers (they sit on the same desk). If you go this route you will have to turn off your wifi and double check you connected the Synergy client to the ethernet connection. Once I did this, there is no lag. This would be the solution if you were gaming or needed to cut mouse latency completely.
You are not going to see any difference in latency between different software that shares a mouse and keyboard because they both rely on WIFI in your situation. Seriously consider using an ethernet cable if you can - the alternative would be to use hardware (KVM switch) which is not a better solution.
Edit 5/26/2017:
It appears this bug has been fixed. The lag over WiFi should be corrected according to a Synergy developer. Leave a comment about your experiences.
Kolob's answer is entirely correct.
Personal anecdote: I was using Synergy over Wifi and the performance was utterly abysmal. Just random lags, stoppages, clicking on tabs would take seconds to update. Truly terrible. Based on his answer I went and hooked up my machines using hard-wired ethernet and everything became butter.
I post this as an answer because the difference between my setup and the one recommended by Kolob is that I used an actual gateway (I used to use it as my wireless gateway before my ISP provided their own hardware.) With this technique, each machine still gets to connect to the internet via Wifi as normal, but my gateway acts simply as a vehicle for providing a separate subnet. (the "WAN" port is not even plugged in -- but the router is providing DHCP and the subnet) That way, I don't have to worry about disabling any existing network, etc., and both can live together harmoniously.
When configuring Synergy, just make sure you're using the IP addresses of the subnet provided by your gateway rather than your Wifi connection. A simple ipconfig or ifconfig should facilitate determining which IP address you want the client to use (the server will be bound to all its IP addresses, so you don't have to worry about it).
On a Mac, make sure to go to System Preferences > Network and then click the little gear icon on the bottom of the list of network interfaces you have (i.e. Wifi, LAN, etc.) and click "Set Service Order". Then drag to make sure your actual ISP interface (i.e. Wifi) is ordered above your ethernet (LAN) interface. Otherwise you won't get any internet on your Mac. (didn't have this problem on Windows for whatever reason)
I had the same problem just a second ago, I tried all the other fixes (other than Ethernet for my mac).
On the host PC (windows), go to Task Manager (Ctrl+Shift+Esc), then click on the details tab on the top, then search for synergy.exe, synergyd.exe, synergys.exe and then right click on them and under the set priority sub menu choose High.