As you record & edit in your session the Clip List down the right end side of the Edit window will get bigger and bigger. The size of this list can impact on the performance of Pro Tools. If you notice Pro Tools is getting slower to respond, then it might be time to prune the Clip List.
Firstly Save your session and then do a Save Session As version of it. That way if you want to get back any of the clips at some point in the future you have a version of the session with them in. To remove unnecessary audio files from the audio regions list, click on the arrow on the right hand end of the Clips top bar and a drop down menu will appear (as above).
Choose Select and Unused. Then go back into the Clip List menu and choose Clear.
and then Remove and all the unused clips will be removed from the session and your clip list will be a little shorter. You may want to choose the Unused Except Whole Files option. This will leave the original files in the list even if they are no longer used in the session as this can make finding original content from the unedited clip easier.