Bubble wand right against most walls can trap bubbles in them allowing for continuous walljump - I figured that out well before I found out you can actually aim the bubble wand up or down (which I learned from the author's notes because I wanted to know how to escape the final dungeon if you start it too early, which I did, at minimum possible equipment). This provides a fun potential speedrun route: walljump up from the third area, right before you'd normally get the phase dash, through the rain, and just get the Big Pearl directly.
This is probably optimal for the 100% run, too, since the Pearl allows rapid equipment switching. You'd probably get the phase dash first in that case, though.