Picker field slow for one user (and only one user) regardless of machine or browser

Hi, 

We have a seemingly exotic issue where either a user picker or custom picker (from a constant) are performing very slowly on one report we have.

Regardless of the machine or browser this users uses, this issue occurs for him, and him only.

Also, pickers of the same type on other reports work just fine for him.

Any thoughts on what might be the cause of this? 

Thanks,

Dylan