  The critical cooling rate $R_{\rm c}$ above which charge ordering is
kinetically avoided upon cooling, which results in charge-glass formation, was
investigated for the geometrically frustrated system $\theta$-(BEDT-TTF)$_2X$.
X-ray diffraction experiments revealed that
$\theta$-(BEDT-TTF)$_2$TlCo(SCN)$_4$ exhibits a horizontally charge-ordered
state, and kinetic avoidance of this state requires rapid cooling of faster
than 150 K/min. This value is markedly higher than that reported for two other
isostructural $\theta$-type compounds, thus demonstrating the lower
charge-glass-forming ability of $X$ $=$ TlCo(SCN)$_4$. In accounting for the
systematic variations of $R_{\rm c}$ among the three $\theta$-(BEDT-TTF)$_2X$,
we found that stronger charge frustration leads to superior charge-glass
former. Our results suggest that charge frustration tends to slow the kinetics
of charge ordering.
