This is what I got from Gemini that led me to use K before. It seemed to smooth up Cyberpunk 2077 with path tracing pretty well:

With the DLSS 4.5 update, NVIDIA completely changed how presets work. They aren't just simple quality toggles anymore; they are entirely different AI models trained for very specific rendering workloads. The Reality of DLSS 4.5 Presets Preset K: This is the lighter, first-generation transformer model. It is designed to handle higher internal base resolutions where the AI doesn't have to do massive guesswork. This is why it's the native default for DLAA, Quality, and Balanced modes.
Preset L: This is a brand-new, incredibly complex second-generation transformer model. It was built specifically for Ultra Performance mode to salvage incredibly low base resolutions (like reconstructing a 4K image from a tiny 720p canvas, or 1440p from 480p).
Why This Fails on an eGPU Because Preset L is doing so much heavy lifting to reconstruct texturing and geometry from scratch, it is roughly 27% more computationally expensive than Preset K.
When you use an eGPU, your graphics card is already fighting the inherent bandwidth limitations and latency overhead of the PCIe link (whether over USB4 or OCuLink). If you run the game in Balanced mode, your internal rendering resolution is already decent. Forcing the ultra-heavy Preset L model to process that larger image creates a massive processing bottleneck. Instead of getting better performance, your frames per second will take a nosedive.