I have a strange issue of mouse lag. It feels sloppy and unresponsive with 0,5 to 1,5 secs of lag. It is worse when i right click to pan the camera around , since the input takes places after 2 secs. Keyboard inputs seems fine, although i haven't use it too much to be totally sure.

It isn't because of GoG Galaxy's interface is it?

Same problem here.

Update: Running the game in windows 7 mode helps reduce it.

I was running into this problem as well. Disabling "Display notifications and access GOG Galaxy when in-game" on the settings menu for Sins worked for me.