I dunno why people say shes slow. She low health and its dangerous but she jungles pretty quickly.
Just looking at her kit, If you ignore her compete lack of sustain and the amount she'd need to return to base, she'll probably come out about as fast as warwick, a notoriously slow jungler who makes up for it by being at 100% hp all of the time. Realistically, even warwick is going to be much faster as she'll have to head back to base early and often, and warwick can start with a damage item like a doran's or long sword while poppy is glued to cloth armor + 5 hp pots. If you can post a path that gets her to level 6 in under 7:30ish (excluding gank/lane xp) i'd love to see it. (all of the serious junglers have paths that hit level 6 in about 6:20 to 7:20 from only jungle xp... although that's just a metric, in practice most will interrupt their route to gank sooner than this.)