I had bought the latest 4090 recently, I do not think synfig rendering is that intensive, but I actually want to ask if you had any projects in the past that required the power of 4090.
I definitely did not buy it for synfig, I was just completing my collection of from the 2080 ti, 3090,and finally 4090 (titan too), and also for Autodesk maya, and blender.
I highly even doubt if 3090 is not overkill, it still looks too much for synfig.
Clearly overkill
There are only a few references to opencl in the code (and it must be compiled with).
I may be wrong, but Synfig does not benefit of hardware acceleration (CPU power only).
Rendering is done on a surface that is pushed through ffmpeg image-by-image mostly.
Surely would love to see hardware acceleration. Infact, I would love to see synfig collaborating with a CAD software, which would open the way for crazy 3d and 2d stuff together.