There's quite a long list of complaints about this on the FileMaker TechNet. (You can join for free; click on "Community" above.)
Most of the blame is falling on list view, conditional formatting, and the CSS Themes that are attached to layouts in 12.
Others have been reporting this in Report an Issue here in this forum.
For More Information see: Scrolling in List view is very slow
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html