In this case AndyClon I don't see how that would make a difference. One camera at a time is all that is rendered anyway. Multipass does not seem to be the issue unless there are gbuffers that are the problem. Even so finishing a render is necessary.
Checking the textures would be easy, just use default to see if that is it. Do make sure, regardless, that you remove master shaders and objects. This can bloat the file and can cause issues with file saving primarily, but good practice anyway. I would recommend all render options be turned off then turned on one by one. If you are loaded up w/ all the bells and whistle you are only adding to a possible resource issue.
The plugin question was not answered tho that I could see. Are you using any? How about advance pack? If you are using primivol it is probably the multithread issue I reported that is still unfixed. It does this very thing you are describing in the render if a custom shader is used as a volumetric primitive. If you turn off multi-threading it should work, but of course take longer.

No need to think outside the box....
    Just make it invisible.