Teleporting a display entity with a high teleport_duration value doesn't move the entity linearly between the two points as expected. It's very inconsistent but sometimes appear to split the momement into two smaller movements, each with a sometimes drastically different speed. Reproducible in both 1.20.6 and 1.21-rc1.

To reproduce:

/summon item_display ~ ~0.5 ~ {teleport_duration:49,item:{id:"minecraft:stick",count:1}}
/execute as @e[type=minecraft:item_display] at @s run tp @s ~3 ~ ~

Observe how inconsistent the movement is with each teleport.