While for most games that I run in i3 the performance is comparable to the same game in a different wm, there is occasionally the rare game that has issues. Skullgirls is the most prominent example for me. When I run it in XFCE, the performance is perfectly fine. However, running it in i3 results in slow, choppy performance. I've tried changing it to run in fullscreen, borderless-fullscreen, and windowed modes, and as far as I can tell, the latter two seem to perform more poorly, sometimes freezing up entirely. Also, when the game exits, it will cause Steam to exit as well (which doesn't occur under XFCE). When I kill the process via commandline, it generally doesn't cause Steam to exit. My best guess is that it seems to be an issue caused by i3's control of the window size, which Skullgirls isn't expecting. (Floating windows still exhibit the issue, although I always set it to floating manually once it starts. I haven't forced it to start floating yet.)

I don't know if this is the appropriate place to ask the question, but I'm not sure whether this is an i3 configuration issue, a Skullgirls bug, or something else. Has anyone seen anything like this occur with other games?

Do you run steam in big picture mode before launching skullgirls? I have problems with some things in i3 when using big picture mode because big picture is fullscreen, and then the game I launch tries to start in fullscreen too so it gets blocked.

No, I was not running Big Picture Mode when I launched Skullgirls. (I did run it previously, as it reliably detects the controller, but that probably wouldn't have any effect.)
Also, I didn't realize I posted this in the general software subforum, rather than in the games subforum, so if a mod would like to move it, I have no objection.

It looks like Skullgirls is using a large amount of cpu on i3, whereas on the other wms I've tested (XFCE and Fluxbox) it uses much less and runs smoothly. I wonder if this is due to i3 not having double-buffering/compositing support.
I'll look into adding compton and checking the results tomorrow.

No luck with compton. Although everything else runs a bit smoother, which is nice. ;-)
I'm not sure why Skullgirls would be hogging cpu like that.