Optimizing game for minimum GPU usage

Hello! In the following days I will need to present my game at a contest, but the computer I am supposed to install my game on (the computer provided by the contest organisers) is probably very bad (especially GPU-wise).
My game takes place in a forest, on a big map with a lot of foliage and a few light sources.
What do I do to make the game smoother, lighter on the gpu side?
(otherwise, the game will run at literally 5 fps, although it runs fine on my personal computer)

Could I use the CPU for GPU-related processes maybe?

I have tried changing the light sources to static but light building takes hours for some reason, and it also looks very bad, so I abandoned that idea.

Can anyone help, please? Thank you in advance!