Kid Flash didn't take anything slow. He lived in the fast lane and threw himself headfirst into everything.
But he couldn't do that with Jinx. She was not fragile, but she needed time to adjust. It wasn't as simple as her switching sides. She had to come to terms with changing, to accept that maybe she could be a hero.
It took time for her to accept herself enough to open her heart to Wally. She needed to take it slow, and for once in his life, Wally slowed down enough it help her through every step.