Assets and performance

I would like to ask if assets manager compare the date of creation of files upon every request or is there any tricky way to create new files in assets folder only when there is some changes made? If it looks for css file on every request and then compares it with the actual version, would it impact negatively performance on heavily loaded apps?

Thank you in advance

This should answer your question about the checks:

http://www.yiiframework.com/doc/api/1.1/CAssetManager#publish-detail