Perhaps, but cpu cgroup for each /service/ won't affect regular apps and their grouping. The kernel's auto-grouping will.
As can be seen from the top screenshot and as explained in the text, all those X apps were splitting the time available to their group, which should have been the one set by the kernel as they aren't services. At least, if systemd had altered their scheduling, it did so in the same way that the kernel autogrouping would have, because the autogrouping would have put the entire X session in the same group, and that's the scheduling behavior observed.
Duncan (on Gentoo, no systemd, mainline kernel 2.6.38, autogrouping on, so what systemd might or might not do doesn't presently affect me in the slightest)