For me it's while it's at its peak. I personally use IR as opposed to XR. This was because I personally have a high propensity for insomnia (to the point that if I have coffee after like 2 or 3 PM I risk not sleeping). So I essentially am prescribed to take it twice a day but rarely go for the second dose. If I do take the second dose I have an insanely productive day but often get no sleep.

Back to the topic of the irritability/temper, yeah it's basically when it's active. Afterwards I'm a bit tired but otherwise totally kosher. For context, I am also on a very low dose of 5 mg. 10mg starts putting me on "talk really fast, move really fast" sort of stimulant buzz. 20mg was utterly uncomfortable. IIRC the higher doses exacerbated the positives and negatives: even better focus, even less patience, even shorter temper.