Sub 18:30 is probably possible if you have a really good run going.

Castle Beach was RIDICULOUSLY CLEEAAANNNNN

Recently discovered a couple of minor time savers.

1. You can move a little as soon as a level starts by holding down buttons. This isn't possible on all levels. On Journey to Ursula for example it just isn't possible. But it is particularly helpful on Castle Beach. This is also possible to do on Return to Atlantica after getting the salad bowl for Tip and Dash - You can begin swimming while the cutscene still shows them swimming into the castle.

2. Swimming to the side when Alana's comb comes out of the clamshell will spawn it on the ground. TBH not really sure if this saves time but it makes picking it up much easier for me.

3. You can just skip one of King Triton's dialogues at the start if you swim down slightly every time you enter the throne room.

4. I don't know if other people know this? I'm probably late to the party. Crabs with lowered claws can just be ran past/swam through. Similarly item stealer crabs can be ignored entirely when underwater. I go a bit more in detail on the guide I made up.

5. What's her name? Morgana? The last boss. She goes on like a search mode if she loses track of you occasionally and it can waste a lot of time. I've started saving a stone to throw at her if she starts doing the "Where did you go" animation to prevent her from turning.

6. Animation for picking up the cake on Castle Beach can be skipped if you jump onto the box from the lower box (not climb) and turn.

7. 100% sure people already know this, but when Tip is playing ring around the rosy with the shark on Ice Floe, sometimes it lines up just right where the shark will headbutt through the ice while Tip is still saying his last line.