I was trying to hunt down this issue for a while and I think it's caused by the server periodically sending out redundant entity move/teleport packets. Every three seconds or so the server will send a packet confirming the location of the entity. This isn't a problem for most entities, but since this packet is treated as a teleportation by display entities, it resets their teleport interpolation progress. This leads to an apparent decrease in speed since it effectively forgets that it was already interpolating and starts again as if it was sitting still.
TrollyLoki
I was trying to hunt down this issue for a while and I think it's caused by the server periodically sending out redundant entity move/teleport packets. Every three seconds or so the server will send a packet confirming the location of the entity. This isn't a problem for most entities, but since this packet is treated as a teleportation by display entities, it resets their teleport interpolation progress. This leads to an apparent decrease in speed since it effectively forgets that it was already interpolating and starts again as if it was sitting still.