Porting a game designed for high-end PCs and next-gen consoles to a hybrid tablet device like the Switch (which runs on an NVIDIA Tegra X1 chipset from 2015) was a massive undertaking. The developers, Panic Button, had to aggressively optimize the engine.
An NSP file is essentially an eShop-style digital distribution format. When you download a game from the official Nintendo eShop, the data you receive is packaged in a format very similar to what the community refers to as an NSP. Doom Eternal NSP - 1fichier - ID 0100B1A00D8CE000