Thanks for suggestions Sickboy.
The client is using 0% CPU and 18.7M memory. There is only one "The withSIX.com client (32bit)" running. Screen cap - https://gyazo.com/73794cee0831b7a8b1ccbb8bfe4ddaa4
I've tried Win 7 compatibility mode as well, no different.
Product version is 1.3.8.
Have tried using the command line option, no different.
Also tried installing the beta version of the client, no different.
Also checked my java version is correct and also tried using Firefox (which supports java in Win10).