TensorRT
The central fact to hold onto about TensorRT is that it is a compiler, not a runtime library you call into layer by layer. Given a model graph and the exact shapes, precisions, and target GPU you tell it about, it benchmarks candidate kernel implementations for every layer, picks the fastest ones for that specific hardware, fuses what it can, and emits a serialized engine — a compiled artifact, not a portable model file. That one fact explains everything else on this page: why building an engine is slow (it is a search over kernel candidates, not a translation), why the resulting engine is fast (every layer runs the kernel TensorRT found to be fastest on that GPU, for those shapes), and why the engine does not travel to a different GPU, a different TensorRT version, or often even a different driver.