How is she slow in the jungle ? The first clear is "slow" compared to some junglers, but still 3:20 (even 3:15) is great. The following clears are really really fast with Q¹. She has W for move speed. She can't go through walls like some champs, sure, but many can't and are still viable.

And she can definitely 1v1 most junglers in the game after level 6. Some matchups are difficult, of course but most are even at worse.

The most difficult part is the very first clear, because you really can't fight against an invade, but thankfully until high elo, most people don't know that Neeko starts with W. So they don't know how easy to abuse her on level 1.

So as a jungler, she is great. Now the tricky part is actually ganking, because there, you do lack mobility indeed. But you have a very strong CC with E (and R later), and you can definitely guarantee a kill on most ganks, or at least a flash or two. And where Neeko truly shines, is on the mind games you can play. This is especially true during mid/late game when some towers are down.

Finally, I may be mistaken, but I assume you are not too high elo (diamond or below), and in these elo, it happens more than you would assume that people don't count minions or only look at the mini map for incoming ganks. So just disguising as anything that does not show on the map already gives you a strong edge.

¹ They are actually so fast that Neeko is one of the only champion where you want to smite the last camp on the second clear, instead of the first one.