Description
In minecraft 1.15, the mechanics of sprinting on the soul sand have been changed, now it takes ≈1.5 times longer. When joining servers with versions below 1.15 (tested on 1.12.2), the sprint on the soul sand slows down, not allowing you to make long jumps (which leads to problems with anti-cheats).
[Demo].

To Reproduce

• Install latest MultiConnect
• Launch and join local (or third-party) 1.12.2 server
• Build the soul sand path
• Sprint (with jumps) on it
• Go to singleplayer
• Build the soul sand path
• Compare the speed

Additional
There are also small sprint (without jumps) difference – MultiConnect client is a bit faster [Demo].