Updated description by GoldenHelmet

Steps to reproduce

1. Build an ice boat track: 2 rows of packed/blue ice with full block sidewalls. OR use water in place of the ice to build a canal.
2. Place soul sand to the side of the packed/blue ice or water, under the full block sidewall.
3. Ride or push a boat down the track/canal.

Expected result

The boat moves down the track/canal without being affected by the soul sand.

Actual result

The soul sand slows a boat that is driven by a player, and stops a boat that is launched by a slime block + piston.
It completely negates the speed granted by the ice.

Original summary
Ice Slowing Boats Down
Original description

I was creating a ice highway (packed ice) going through the nether for 17k blocks. After creation, whenever I try ride the highway at full speed on a boat, it randomly stops me. Even when the chunks are fully loaded. It acts as if some of the ice blocks are regular blocks. I tried replacing the ice, but it still continued to act the same way. This is on a realm that is 343 megabytes big

How to reproduce the bug?

i dont really know. Probably just make a massive path of ice, going about 27k blocks in the nether, and try ride it with a boat at full speed.