Horse working in an outline: getting slower

My five-year-old mare and I have just got to the point where she has started to work in an outline and step under herself more. ❤️ I'm wondering if it's common for a horse to feel slower or more sluggish once they've started to work in an outline. When I first started riding her she had no muscle and poor fitness, so she had to take a certain number of rest breaks, but now she has muscle and fitness. Will it be the same once she's working in an outline? Will she feel just as forward and lively then too? Because she's a lively horse when she's not working in an outline.