I have been on Focalin extended release before. It releases half of the drug about four hours after the initial instant release. At this point your plasma level peaks at a little more than half of the first peak. (A psychiatrist once showed me a graph in his reference manual.)
This lower and prolonged blood level could result in a "dragged out" stimulation. Irritability and impatience are common effects with drugs that inhibit the reuptake of norepinephrine in the way that (dex)methylphenidate does.