If you use the /entitydata command on a repeating command block while you're in survival mode, it causes extreme lag while updating the entities, compared to Minecraft 1.8.
What I expected to happen was:
I used this entitydata command in Minecraft 1.8 for my maps. I made a map in the 1.9 snapshots. I used an entitydata command to make my mobs tougher against hand attacks.
What actually happened was:
When I changed to survival mode, the world started to lag a lot, causing mobs to move and attack very slowly.
Steps to Reproduce:
1. Have a few entities in your world (around 50 entities should be enough!)
2. Place a command block.
3. Set the command block to Repeat - Uncondictional - Always Active
4. Use the command: /entitydata @e[type=!Player] {AbsorptionAmount:1}
5. Change to Survival Mode!
6. Try to mine a block or hit a mob. You should see that the mob movement is really laggy, and the blocks you mine sometimes reappear. This didn't happen in Minecraft 1.8, unless you had hundreds of entities or something. If this doesn't work try adding more entities.
Have you set
/gamerule logAdminCommandstofalse? If not it might be caused by the long log messages