Stuff to ponder about
How much transcoding do you do ?
'Rendering' is the terminology used when you create animated overlays like tickerbars, greenscreen effects , crossfades etc. you don't seem to be doing any of that in your movies. Render operations are doing pixel operations for which the gpukicks in.
What you are doing is transcoding. Take a chunk of video recorded in format x, and spit it out in format y.
Your camera films in format x. Your output is a different format ? What if you keep the format the same (input and output for,at identical) then the encoding only happens at the intersection of two clips. Most of the other data is simply copied over. Like merging two files. There may be a tweak where you can tell the editing tool to adjust your cut points so they always coincide with an i frame. In that case there is no rendering involved at the handover point as a frame sequence always has to start with an i frame.
Do you know what the gop format looks like for your input format and output format ? If the gop lengths are different then encoding becomes time consuming.
What other effects are you running ? Color correction ? Audio leveling ?
Here's another thing to think about. If you watch commercial broadcast in HD. Let's say a news channel. They have the live studio feed, overlay a titlebar, have an animated scroller at the bottom , may inject an animated background and when interviewing a live reporter do picture in picture.
This is all done by a single computer , in realtime.
How do they pull that off ? By skipping decoding/encoding.
The live video comes in as an uncompressed format so the computer does not have to waste time expanding the stream and then re compressing it. The datarates are not that high for a modern computer. Overlays (tickerbar etc )are run on the gpu. Assembling the output video is a chunk of or operations using masks take live feed, apply mask (set certain areas to black). Then take overlay anything that is not black in overlay gets copied over to anything that is black in stream. Done.
Very simp,e bitswap operations.
When the final video is ready , then does it get encoded , once in the desired transmit format.
You may be able to do such thing also. Your camera has a live video output over usb ? Meaning if you plug it in and launch a video capture tool it sees the live feed ? You may want to try to capture, using a laptop , directly in an uncompressed format , or a format that only uses i frames. As opposed to usng the in camera encoding. You will have very large files but there is no deco pression step anymore. You have raw uncompressed video and audio.
That may edit much faster than the native format from the camera. Edit in vegas, save as uncompressed , and let handbrake do the encoding .
Something worth a try.
My gut says the computer is spending al its time decoding/reencoding.