Hello. It seems there's some problem in particular with SVP implementation of RIFE interpolation and/or VapourSynth - it runs out of CPU on the threads it uses for high FPS output.
(That at gpu_threads=8... could use 16 there but filter does not support it.)

720p 60 x2 works. So does 1080p 30 x2. 1080p 24 x3 runs into a CPU cap, getting 105 FPS output. 1080p 60 x2 same, getting 85 FPS output.
Encoding is *much* faster than this.

Hardware: GeForce 3080 (OC) and Ryzen 5950x.

Can anything be done to make it actually not run out of CPU?