When the 'Age' tag of an 'AreaEffectCloud' entity is modified using the 'entitydata' command the 'Effects' tag of said entity will be duplicated.
This causes the data tag to get longer and longer.
When the 'Age' tag is modified every tick using command blocks the tag will get so long that the game starts lagging behind because the data tag is so long.

An example of the duplicated data tag is inside the attached file.