The bug

Setting the scale attribute on the ender dragon causes extreme TPS loss. TPS loss increases the higher you set the attribute. Even just setting it to 10 makes the game almost unplayable with more than 80ms per tick, while other entities have no issues with higher scale values.

Also see the TPS graph on the attached screenshot.

This happens both with the naturally spawned dragon in the end and one spawned manually in the overworld.

How to reproduce

1. Go to the end or /summon a ender_dragon
2. Open the TPS graph with F3+2

3. /execute as @e[type=minecraft:ender_dragon] run attribute @s minecraft:generic.scale base set 8
   

4. Watch the TPS and FPS die