Matt's words
Since they only had less than 48 hours to train, they had to increase the intensity and focus on where they failed to improve.
For Isla, knowing that Matt was leaving her behind was a big worry that hung over her head, so throughout the training, she kept active and did her best.
After all, she was Matt's teammate at present, but Matt already outnumbered her by over 1,000 Power, which made her feel pressured. Add to that the issue of her believing she was a liability and the worry that Matt would find someone more useful and kick her off the team increased, so she didn't want to let her guard down.
She wanted to find a way to show Matt that she wasn't a burden, but her traumas and low status might not allow it.
Matt naturally noticed the concern in her but didn't mention anything to her. He simply trained hard during these hours with her, focusing on teaching her various things so that she would learn how to be an assassin.