At times, your system may seem to be spinning its wheels. A peek into Activity Monitor (found in the Utilities subfolder of your Applications folder) can offer clues as to what's going wrong. Activity Monitor scans your computer to determine its current CPU load and helps visualize other system tasks related to RAM, disk use, network access and overall energy efficiency.
You can discover which file that is by selecting the process name and clicking the "i" info button. When the info pane opens, click the Open Files and Ports tab.
There you'll find the path to any file causing an issue. As a rule, you'll probably want to delete that file, which hopefully isn't too important. As the most likely candidates are generally media files (in my case, it was a corrupt movie recording), you may want to use a file converter to create a safe copy before deleting the problem file.