Why is my CPU working super hard when I render a project in Kdenlive and have a good GPU? I have an NVIDIA GeForce GT 730 GPU (...)
A Nvidia Geforce GT730 isn't "good" and never was, even when new some 8 years ago.
And it doesn't supported neither the codec required for video encoding (NVENC) nor the parallel computing platform (CUDA) as explained in the Daniel B's answer https://superuser.com/a/1748240/653259 (with links).
HP Compaq 8000 Elite CMT PC with an Intel Core 2 Duo E8400 (2) @ 3.000GHz CPU
This CPU, Intel® Core™2 Duo E8400 is even older yet "good(ish" in its time, some 14 years ago, which in computational terms is an eternity ago. When your CPU was released Ubuntu was still in its infancy, that's how old it is.
GPU acceleration in KDEnlive started with 20.08.20. Assuming you installed KDEnlive from the official repositories you should be running version 20.12.3-1 which indeed supports GPU acceleration and a tad less experimental than the aforementioned version. However, as already commented by @Cpt.Whale,
Make sure that you're actually using a format that supports GPU acceleration like NVENC. Kdenlive also still processes a lot of effects on the CPU only, like color grading or chroma keying.
So, there a are many "moving parts" here to give you a definitive answer.
To know: The CPU is always used regardless of concurrent GPU processing and given its specifications having it maxed on both cores IS to be expected. But the GPU may not be used at all even when the software and driver support it, depending on the codec being rendered and even KDEnlive settings.
In conclusion: You're using an old, almost obsolete system (and de facto obsolete by some metrics) for video rendering, arguably one of the more demanding tasks you can do in a consumer grade contemporary PC, let alone in one so old as yours. You should have realistic expectations.