What are.thm?.thm is used as an abbreviation of the word thumbnail (miniatures).thm files are basically a miniature of a video (.mp4), as image format. It is a frame of said video, a random capture of a specific moment of that video. When you want to select a video with the GoPro App (both on Android and IOS), the App shows a mosaic of mini. THM are thumbnail files, I assume for the LCD playback feature. LRV are very low quality versions of your videos. They are used as the wifi preview. I assume this is why there is a 2 second delay, those files need to be created. THM files are JPG thumbnails used by GoPros as photo previews. Both are used by the GoPro mobile apps. Both are used by the GoPro mobile apps. You can safely delete both filetypes–they'll be regenerated from the original MP4 or image file if needed.
.LRV files do have a purpose!! If you have a Hero 3 Black Edition or a Hero 2/Hero 3 (White & Silver) with the latest firmware, you may have noticed .LRV and .THM files appearing on your memory card. For a more detailed explanation on what these files are, please refer to this thread: http://www.goprofanatics.com/gopro-hd-hero3/3896-what-lrv-thm-gopro-files.html .LRV or low resolution video files can be very useful for those of us who edit GoPro footage on lower performance hardware (heck, even high performance computers can't handle the insane frame rates of GoPro files). First thing's first, you will want to copy both the high resolution video file as well as the low resolution file to your computer. Place your low resolution video file in a separate folder (you can name it proxy or low-res files for example). You will notice that the .LRV file isn't even recognizable by your computer and as a result, you can't view the video file. Don't worry, we'll address that now. Go ahead and change the extension of the .LRV file. You can do this by right clicking on the file and selecting rename. Instead of '.LRV', change it to '.MP4' and now you will be able to open the video file. You will notice that this is definitely a very low quality version of the high resolution video you originally took. The reason that we created another folder to hold the low resolution files is because now you have both the high res and low res files named the same. Now you want to open your editing program and import the low resolution video files. Do all of your editing with these files (cutting, transitions, adding music, etc). I would suggest doing your color correction in the high resolution version. Once you're done editing, some editing programs will have the ability to replace files with other ones. This is when you replace the low resolution files with your high resolution files. All of your edits will carry over and then you can go ahead and do your color correction and export your final video! You will save a lot of time by following this technique. You will also avoid the frustration of editing high resolution video files that your computer simply can't handle. If anyone has tried this technique, please post up the editing program used along with a step-by-step process since I'm sure a few of the steps may vary slightly based on the editing software used. I have only tried it in Adobe Premiere Pro CS5 and will be creating a step-by-step tutorial on how to do it.