Soil walk speed is 87% and stone walk speed like granite and marble is 93%. The difference seems small on paper, but in reality it's night and day. This is made very apparent with bionic pawns. One of my colonists has 195% Moving stat, yet he moves almost as slow as regular pawns on the grass / soil, but starts flying past everyone else on a stone surface, like he should be doing everywhere. Why do they slow down to a crawl on soil surfaces when movement speed is reduced just by 6%?
1: Increase Moving stat by as much as possible to make it easier to see the difference. (bionic mods make it easier, like epoe, rbse etc)
2: Test the colonist speed on soil and stone surfaces and see for yourself.
Edit: I didn't know the tall grass slowed them down this much, but it looks like it does. It's a bit confusing since it says walk speed 87% with and without the grass, but it's not really a bug I guess.
Since there was also a tall grass it's not a bug I think. Thanks for reporting anyway.