When does the FFMPEG video conversion happen? When the user uploads the video, when a user first views the video, when a scheduled cron task runs? Preferably, an option should be provided for the admin to select when the video conversion should happen. Do you provide that option? If not, do you plan to provide that option, and if so when?
For those who are interested in video conversion when the user uploads the video, this can have the disadvantage of discouraging users from uploading 'cause of the extra time to convert in addition to the time to upload. Also, the script may time out and not finish if the server's max execution time is set low.
For those who are interested in video conversion when the user first views the video, only the very first user is affected by the slower page loading due to the conversion.
For those who are interested in video conversion by a scheduled cron task, this option needs to be coupled with the "when a user first views the video" option so that a user can view a video that has not been converted by the cron task that has yet to run.
Therefore, giving the admin an option to select from the above 3 choices is ideal since only the admin knows the server load of his/her site.