Because I have image sequences and it is recommended to keep them at 1:1, I will see if I can remove the background in Natron then render the images as as a video file.   So I can import & proxy all video scaled to 1:4.  BTW can cinelerra mask with some sort of edge detection by any chance?  I have not used a green screen behind my subject and the background and subject have some overlapping hues and exposure so chromakey is not a good option.  Otherwise I would have to change the mask almost on every frame...

My Settings->Preferences->
  - Playback A TAB:
    - Video Out:
      - Play every frame = UNCHECKED
      - Video driver = X11
        - Use direct x11 render if possible = Checked
  - Performance TAB:
    - Use HW Device = none
    - Project SMP cpus: 4

My Laptop works good enough in that condition.

If you don't change type of extension (or don't change the Scale Factor) for proxy but change type of coded (Compression) you have delete or move your proxy files to another folder. For example, if you have chosen FFMPEG | qt, Compression=png.qt and then you want to change from png.qt to mjpeg.qt, Cinelerra thinks that proxies have already been created. Cinelerra-GG don't know the difference between png.qt and mjpeg.qt.

My project is only HD, and now all the original media seems to play ok in compositor without crashing so I decided to just use the original files as is.  I had even tried to load my image sequences, select a non alpha proxy, (figured it might be useful for some editing) then create proxy upon loading my videos with one of the alpha codecs, but I just made messes in my folders, like some media still got double proxied, lol! @phylsmith2004, I can confirm that attempting to selectively choose what to proxy seems not to work so good at the moment...

I found out that the very latest version of Cinelerra-8 HV, (source) seems to have GPU acceleration enabled, of course many computations still require the CPU, but I was wondering if perhaps this will be included in Cinelerra-GG?  I do see the option for Cuda, is that the same thing?  I am making more space on my / drive to download nvidia-cuda-toolkit.  (As was mentionned, there are limits to what is accessible, doable with the community &with NVIDIA...)

I am glad there is a dedicated community!

10/23/22 - Cinelerra 8 GPU accelerated rendering. Faster GPU accelerated playback. Write output to command line ffmpeg.

....
Was last tested on Ubuntu 22 x86_64.

For GPU acceleration, it needs the proprietary Nvidia X11 drivers & CUDA
libraries.

....

Unfortunately, all Open Source NLEs have problems with GPU acceleration. Only big companies with lots of experienced programmers in video/graphics/OpenCL/OpenGL can get good results (as is happening today with AI, I think it is the new revolution in editing). Plus CinGG during the workflow on the timeline has some forced single-threaded steps so there is not much advantage in switching from a single core CPU to a multicore CPU. I had only a small improvement, due to the higher processing frequencies, going from an Intel Sandy Bridge 2720QM (11 years old) to an AMD Ryzen 3700X (8 cores, 16 Threads). The freezes I had before continue even now....

On the term "digital intermediate" there is a little confusion because of the legacy of the film days. Back then, off-line editing (which then went through an intermediate) consisted of creating a low-quality copy of the daily, i.e., a proxy, both because the development lab took less time and also because the editor could work even on low-quality copies, since he was only doing the editing. So intermediate=proxy!
Today, with the digital workflow, off-line editing is done with high-quality intermediates taken from film or poor-quality sources, which at least do not further degrade the data during washing (especially color data...). But current NLEs, including CinGG, do not need off-line editing, so we resort to high-quality digital intermediates or proxies depending on the needs of the project. If we also do Color Correction and Compositing/VFX high quality intermediates are recommended, if we have efficiency issues on the timeline proxies are recommended. So Intermediate NOT proxies.

mpeg.mpeg has such a low bit rate that it runs fine on any hardware, that is why it is set as default in the proxy option of CinGG.

Doing (temporary) rendering of a part of the timeline is the "background rendering" option, which creates a sequence of still images. Every variation you make or filter you add makes the rendering repeat all over again. see:
https://cinelerra-gg.org/download/CinelerraGG_Manual/Background_Rendering.html

When "Rescaled to project size" is checked, the size of the video data to be computed will always be as the format project.
The size of your video is scaled down of the Scale factor BUT, then, it is rescaled up to the project format.
For example, your format project is 1920x1080 and Proxy's Scale factor is 1/4, then your videos will be scaled down to 480x270 and then up-scaled to 1920x1080. For an old computer, this option is not recommended.

When "Rescaled to project size" is UNchecked, the size of the video data to be computed will be reduced of the Scale factor.
The size of your video is scaled down of the Scale factor.
For example, your format project is 1920x1080 and Proxy's Scale factor is 1/4, then your videos will be scaled down to 480x270.
It works as if the format project were 480x270 (and really it works so).
The good thing is that your computer will be faster. The bad thing is that some Effects (Plugins) doesn't work as expected because they use pixel units. An example for that are Title, Blur plugins. A workaround is needed for that.

What I found confusing is that the many options for both transcoding and proxies seem identical except for the scale option, hence trying to wrap my head around the difference.  How I remember it in Cinelerra for Grandmas manual,  the terms intermediate codec, at that time there was DNxHD and the mjpeg type, were almost used interchangeably with proxies since we still had to point Cinelerra to the original files for the final render.  I am surprised to read that  with good codecs, you can now drop the originals!

My main types of files; TIFF sequences (which could not play in compositor, would cause Cin to crash) & MTS video originals (interframe long GOP) were all transcoded and replaced with DNxHR_HQ  but upon testing with effects playback is slow esp the video ( 3-4 frames per second).  

(To complicate things, I was hoping to try a few things like the painting effect in Natron with the edited footage (Natron works with many sources in Rec.709 space) but I might save that for another project.)  

So maybe I should get my originals back in project and just proxy them with DNxHR_lb so I can switch to the originals for the colour correcting.  If I didn't need an alpha channel, for masking & chroma key effect, I would have liked to use the mpeg.mpeg proxy, it seemed to play smoother.  Can we still render small portions of a project in a float format and bring it back on the timeline if needed? 

Good clarification - definitely not the same things! But in any case it would be less messy if both Proxy and Transcode files were put somewhere else.  I did log a BT:

https://cinelerra-gg.org/bugtracker/view.php?id=626

Currently these files MUST be in the same directory as the original files - the original design goal was to make it as fast as possible to switch from viewing the proxy file back to viewing the original file (the P/S switch in the upper right hand corner of the program window) because you would only be using Proxy if you needed speed in the first place.

I would argue that you would also use proxy clips if you want smooth playback probably, but not necessarily, the same thing.

As for transcoding, it only needs to be done once and afterwards the original files could be deleted if desired.

We seem to be confusing transcoding with proxy generation. Transcoding is converting clips to a different working format, clips that can then be used in the place of the original clips. Proxy clips are low resolution clips to be used alongside the original clips, as temporary substitues but not to replace them. Both should help with a slow computer, in some cases proxy more so than transcoding.

"I'm hoping that some of the transcoding, effects etc can make use the GPU processing- I have set the video driver to x11-openGL"

Although the video driver X11-OpenGL is the best for video playback because it uses PBuffers and shaders to do video rendering, there are some plugins and transitions that can not use OpenGL and will be using software instead of hardware which slows down playback.  Camera and projector operations use OpenGL,  You can change Settings->Preferences, Playback A tab, Video Out section to uncheck "Play every frame" but for me, missing playing a few frames just confuses me.