.pek - These files contain the actual visual representation of the audio waveforms you see in Premiere’s timeline and Source Monitor. They used to be stored in the Media Cache Files folder, but were given their own dedicated Peak Files folder in Premiere CC 2017.

.mpgindex - These index files tell Premiere where whole ‘I’ frames exist in some MPEG-based files which helps Premiere understand how to interpret the non-whole frames that exist in a temporally-compressed formats. If that made no sense to you (it’s a tough, technical topic), check out this Wikipedia article: https://en.wikipedia.org/wiki/Video_compression_picture_types

.mxfassoc - Some MXF files have media attached to them or multiple media streams and this is a file that keeps tabs on those media associations.

What does ‘Clean’ do?

Back in Premiere, one of the first things your eyes will lock onto in the Media Preferences is a ‘Clean” button. One might come to the reasonable conclusion that if you click this, it will purge your entire Media Cache, but you’d be wrong! Clicking Clean actually takes a few moments (depending on the size of your Media Cache) because it’s going through and looking for every file you’ve ever imported into Premiere (using those .mcdb files)! If it can still find the source media file, it keeps any associated cache files. If it can no longer find the file, it deletes them.

How do I REALLY purge the Media Cache? Should I? How often?

To truly dump the Media Cache, you have to do it the old-fashioned way - manually. First, quit Premiere. Then, in the Common folder, highlight Media Cache, Media Cache Files, and Peak files (if it’s there), and delete them. That’s it! When you relaunch a project file, Premiere will automatically regenerate the necessary cache files for the media in that project. The only drawback to this is that if you re-open a particularly large project (i.e. hundreds or thousands of clips), this could take a few minutes.

So, why should you? Well first off, extra hard drive space is nice! Especially if you’re purging files that you no longer need. It can also be a troubleshooting tactic for a few situations including if you ever run into the dreaded yellow ‘Media Pending’ message for a clip in the Source or Program Monitors or have clips not playing audio or displaying waveforms. As for how often, there’s no real recommendation here. When your hard drive is filling up? After wrapping a giant project? It’s up to you.

Can I use this to boost performance?

Merely deleting the Media Cache isn't likely to boost performance unless your hard drive was dangerously close to full. However, you can optimize Premiere’s performance a bit by using the Browse command (in the Media Preferences) to change the Media Cache locations. If you redirect the folders to a separate (ideally dedicated) fast drive, you may see a slight increase in performance. This may also be an ideal setup in collaborative environments (assuming very fast network storage) to prevent cache files needing to be generated on every user’s system. Keep in mind that this setting is not retroactive, so it will have to rebuild the cache files.

Bonus lesson: The Project Manager and Audio Conform Files

Additionally, Audio Conform files are referenced in the Project Manager as optional files to consolidate along with your media. Choosing to do so will prevent Premiere from having to re-create potentially tons of large conform files when you re-open the consolidated project another another system! Of course, the drawback is you’re consolidating a lot of extra media with your project that Premiere can always re-create if necessary. It’s up to you!

There are two new things to know about the Media Cache after the Premiere Pro 2017.1 update, both of them quite important and useful!

Media Cache Management Policies

You can now tell Premiere to auto-delete Media Cache files automatically by one of two simple rules: after a certain amount of time or after the cache reaches a certain size. The settings are found under the Media Cache settings in Premiere’s Preferences.