When joining any server that is on 1.13.x; Walking instead of sprinting (or doing anything else) will make the player movement feel like you're moving slightly slower than expected via 1.12.x and other versions.

How to reproduce:

1: Play on any server that supports 1.13.x clients,
2: Try to walk anywhere you want and then basically compare it with another version using multiconnect itself.

Cannot reproduce. I tested a real 1.13.2 client against a multiconnect client on a 1.13.2 server, they took 23.66 and 23.64 seconds respectively to travel the same 101 blocks, within the margin of error. Check your FOV setting, that can affect human perception of speed.

Tried lowering the FOV to normal instead of Quake Pro; The speed feels fine again in 1.13.x instead of slowered via human eye-sight.

In short = Issue is invalid.