Time Spy is an older benchmark and doesn't scale well with Core Ultra CPUs.
Furthermore, memory makes all the difference in CPU scores, and it seems that with the i9-13900HX you used 6400MT/s modules while with the 275HX you used 5600MT/s.
With VBS disabled, stock memory (5600MHz CL46) and stock CPU, I reached up to ~17700.
Everything remained the same, but with CPU undervolting and an IETS GT600, the CPU score increased to ~18500.
And finally, with memory adjusted to 6400MT/s CL36, I now reach up to ~21900.
I've seen scores of ~24k with faster memory.
Regarding the GPU, its score with the 4090 is good, slightly below its maximum overclocking potential (up to ~24.5k), but its score with the 5090 is not so good...
With a well-tuned overclock, it should be able to exceed 27k and perhaps even 28k with a good cooling pad