i think that the way that rek sai's speed buff works is totaly incompatible with her theme

So rek sai's main theme is a land shark coming at you with high speed just to rawage you along with the ground you stand on and its arealy nice theme (totaly ripped out of fizz thou)

But the way that her speed boost works being a passive boost on her burrowed form is totaly not portraying that propertly if you look at the theme rek sai is a chaser yet she isnt as fast.
Also chaser theme means that she shouldnt be good at running away so much.

To while we have that tunnel and stuff i think that move speed buff should be tweaked to optimize it for chasing while being less usefull when running away from enemies.

The easiest way to look at it is to take a similar aproach like vayne. She has where her passive only works when she chases an enemy moreover this leaves some additional space in power budget to make her ult feel even more usefull by adding a multiplier to amount of movespeed we gain from that burrow.

Looking at the lore teaser and stuffs its easy to assume that she was supposed to be good when chasing but that movespeed passive totaly does not fit as it stands.

While it is satsfying to use i dont think that its the kind of satisfaction that was supposed to be there. So if we speak about the theme, passive speed boost kinda wrecks it (also if we speak about her filling any niche with her kit instead of being another ball of stats that does everything)