Hello Axel,
I tried to replicate the slow-down you depicted using similar test environment under VS 2008 but unfortunately to no avail.
To progress in our investigation, is it possible for you to post the markup of your page/user control in this forum thread (using the 'Format code block' dialog from the upper right corner of the forum editor) to investigate the matter further? I will use it in a simple test project in order to reproduce the abnormality and advice you how to address it.
Best regards,
Sebastian
the Telerik team