Express and NestJS sit at opposite ends of one axis: how much structure the framework gives you. Express gives almost none, on purpose. NestJS gives a great deal, on purpose. The right choice is about your team and your timeline.
Node.js and Python are the two default choices for a backend, and the decision is usually made on what the team already knows. That instinct is not wrong, but it is incomplete — they have genuinely different strengths, and the workload should sometimes override the habit.
Node.js is not slow. Slow Node apps are slow because something blocks the single-threaded event loop, or the database is untuned, or async code runs in series when it could run in parallel. Seven fixes, in order of impact.