Rendering blank video is taking forever

[Toni230236088p4i]

I have a 7sec clip I want to put at the start of a 15min video, except everything after the clip is supposed to be blank/transparent.
I'm trying to render it using Media Encoder as a .webm file (to include the alpha channel), however even though most of the video is just transparent nothingness, the render times are very long (around 14h).
Is this render time accurate or will it render the blank part faster once it gets to that? Is there a way to shorten the render time while still exporting it as a .webm file?

[Rick Gerard]

I cannot think of a single reason to render 15 minutes of transparency. If you are going to the web, edit the two clips together in Premiere Pro and export the final for the web. If you are going to an editing or compositing app, just make a cut or stack the layers. 

If you need a 15-minute overlay, create a graphic and bring it into Premiere pro. 

I'm also not sure why you are using WEBM instead of H.264 if you are going to the internet. The last study I read had a data table that showed WEBM took about twice as much CPU power to decode and playback as H.264.  If you send a WWBM file to YouTube, the compressed files that YouTube (or Vimeo, or any other streaming service) will not look as good as one sent using their recommended format and compression settings.

[Toni230236088p4i]

Thank you for the response!
What i'm trying to make is an overlay used for streaming, an alert specifically. Since I want it to pop up every 15mins, an easy way to achieve that is by including a 15min transparent part to get that pause in between.
In order for it to be an overlay I need to use some file type that includes the alpha channel (I could use QuickTime as well but then the file would be much bigger - however if that solves the time problem i'm willing to accept that), that's why I went for .webm.

[OussK]

Usually i use Quicktime Apple ProRez 4444 when i want to export a video file with Alpha channel so try that, even u don't think the file size will be huge as it's only a empty alpha channel. 

[Rick Gerard]

I don't know what kind of software you are using for streaming, but almost all of the software I know of supports still images as overlays, and you can set timers to have them pop up.  Looping a 15-minute video seems like a very awkward way to approach the project. I don't know of any standard web-based media players that support Alpha Channels, but you may be able to make WEBX work with your application. It will take a tremendous hit on your CPU to process it. 

I would explore your streaming software and see what options there are. There has got to be a better solution than rendering a video with 15-minutes of transparency between a message.

I would suggest you explore the documentation that comes with your streaming software or service, check out their blogs, and study the user guides. 

If you really want to render a 15-minute video with a 7-second announcement at the start and nothing else, then I would set up a 7-second comp with your announcement, and render that, or turn that into a MOGRT, then open up Premiere Pro, create a 15-minute sequence by setting an out point, add your 7-second announcement, and render your movie from Premiere Pro. It will render much faster than you can render one in After Effects because it uses an entirely different method of copying pixels.