Um, is there not an option? I know for a fact that I’ve seen my FPS dip up or down depending on which client had focus. For me, that happened by default.
Oh, I play in fixed window, and use an auto hotkey script by Kuroro Lucifer to move my clients around.
Also, Eve Client Switcher by Laurinius might be worth checking out.
And yeah, it’s nice. I have 4k TV, so I can perfectly tile 4 1080p clients on it.