If a TnT entity has -y motion, it will perform a raycast on movement which is usually utilised for entities to calculate their accumulated fall damage, however this check is not necessary for TnT which is unaffected by fall damage. This raycast produces severe performance issues in TnT projectiles travelling at high speeds as the raycast will be projected over a very long distance for each tick the TnT moves despite achieving nothing.

The relevant code can be found in Entity.java