I also had this problem, tried switching for "Focus-follow mouse" now and see the results, moving the mouse into the window is much quicker now anyway.
Other then that i have another question, regarding PiP. I can't get it to work, is there anyway to change the keybindings for it, because if i do just like it says on the webpage it's just targeting my partymember as that is F2,F3 etc is for in-game :P