Render only a certain part of a frame, reuse the rest of the frame from the frame rendered before I have ~800 frames which are technically still. In these 800 frames, only a small section of it moves, is it possible to only render the first frame fully, then only re-render the small moving part of the 799 frames left and reuse the rest of the image from the first rendered frame?

I want this rendered for 800 frames



Only this part moves for those 800 frames
You can solve this by separating the foreground and background elements and rendering them separately, then using the compositor to re-combine them.

Result of this approach

First, render only the very background - the object(s) that is behind everything else, and save to file:


Then, make sure you set the background to transparent.

Then render the static foreground elements (the elements above everything else that aren't animated) and save to file:


Now you can render your dynamic, animated element separately from the rest. Make sure you render to an image sequence of PNGs with transparent background:


Now all you need to do is open the compositor and use alpha over nodes to composite the static and dynamic elements, then render it all out together to one video file (or a new image sequence):


Alternative node setup using Z combine:


You can find the blend file that was used for this answer here. Good luck!