Refining Speed
Perhaps it was because of their experience or just the concentration of Dimensional Energy was thicker in that Dimension, not only Irina but Frederic, Clark and even Caroline all showed up as a middle Stage 1.
Although the gap between early and middle Stage 1 wasn't that huge, the quality of their refined energy and the activation of their Concept would be a lot smoother. Moreover, they could accommodate another Concept again.
Aldrin believed that he could also accommodate another Concept after reaching late Stage 1 but he's putting it off for the moment since he couldn't even show it all off.
As for the other trainees, they're also progressing steadily. If nothing else happened, most if not all, would reach middle Stage 1.
That's kinda slow if compared to Aldrin or the two late Stage 1 Energy Refiners from the other cities but that's actually considered average among the majority.