Transporters seem to have issues when unloading, it takes too much time for them to "think" when doing so, even if they are in the correct location and position.

This is the reason why most players just drop the units from the transporter instead of just waiting for them to scratch their back and then lower the units down slowly.

There was a nerf a few years ago because transporters were loading units too fast in combo with "gnat com nap". That proved to be "hard to counter". Since that modification, they wiggle and wobble before picking the unit and I think this also impacted and complicated the unloading phase.