Sony Vegas Pro 12 - CPU usage depending on project

I render videos for Youtube at 720p and 29.970 FPS (NTSC).

When rendering a project with effects, multiple video splits, audio splits, text media, and many video/audio tracks, I notice that my CPU Usage fluctuates between 70% - 90%, NEVER hitting 100% CPU Usage. The rendering time for a 10 minute video is ~2.5 hours.

When rendering a project with just a clip of gameplay recording (not effects, no edits, just a video and an audio track), the CPU Usage constantly stays at 95%-100% and the rendering time for a 10 minute video is ~20 minutes.

I'm very confused as to why my CPU Usage differentiates when rendering different projects. Thank you for replying in advance.

RAM: Kingston HyperX Blu 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
GPU: Galaxy NVidia GeForce GT430
CPU: AMD Athlon II X3 455 Rana 3.2GHz
HDD: 5400 RPM (I'm unsure of what it is)
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  1. Best answer
    Probably because your bigger projects end up memory/IO-bound instead of CPU-bound.

    Once you run out of RAM, the OS starts using virtual memory and performance goes down the drain no matter how fast your CPU might be.

    You also want the standby + free total in Task Manager to be high enough to cache a fair chunk of your input media or you will get some more IO performance hits from material having to get re-read from storage more often.

    In either case (virtual memory and insufficient spare memory for caching) you should see that your HDD activity LED is almost constantly lit. If that is the case, you may want to upgrade to 16-24GB RAM.
  2. If you still want to know I tell you whats going on...
    Well, if you have turned on your GPU accelerator on "preferences/video" in Vegas, it means that any process that uses CUDA technology (and you have 96 CUDAs in your GPU) it will be used. Thoses processes could be a group of effects and transitions that come with SV 12. So if you don't use any of these processes the whole work lies in the CPU only. If you test it rendering a piece of footage (with no effects at all) and trying with and without CUDA, the time will be exactly the same. Then if you apply a bunch of effects (the GPU accelerated ones) to that footages, CUDA starts to work, and ironicaly you can check the the uses of the CPU goes down from 90% to 60 or 70 depending on the number and type of effects you applied.
    And if you really wants to compare the same process with the same effects rendered ONLY by CPU, you have to go to preferences and deactivate the GPU. You will see that the render now will take 5 or 6 times comparing with the previous rendering with the GPU.
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