[Amadou]

Your monitor's electronics may be doing more processing at 30Hz (like frame-doubling) than at 60Hz thus adding latency that is not inherent in the lower refresh rate.

I used the T221 at 30Hz in a multi-monitor system with the other two monitors at 60Hz. I could drag a window so it straddled the 30Hz monitor and a 60Hz monitor. The DPI was severely mismatched so it wasn't very useful to straddle like that, but movement across the monitors was not obnoxious, not terribly fluid, but not annoying either.

I'm sure different people have different tolerances for latency, but I tend to think my tolerance is pretty low.