GB Messager : slow load on sites with users and items

If you enter a site with existing items and users (can't say how many), the gradebook messager takes a while to load. The popup is shown empty and is only after a few seconds that you get the see the expected options:

For example:

https://trunk-mysql.nightly.sakaiproject.org/portal/site/IPMF_245O_8211/tool/903adc94-9fde-4579-8eb3-5298e0fcec5b/

It seems something is cached after first access, so you can try on a different browser if you have already accessed the tool before.

EDIT: The blue square is quite a bit off on Firefox, but this only happens before the options are loaded and the focus is on the input.