Building the NavMesh is time consuming, but once it’s done, it’s done. It’s supposed to be efficient.
The re-cast agent it comes with offers optimisation and customization options. And a debugger.
There’s always some fiddling and there are modifiers where you add or exclude sections. And it can be made dynamic - probably much slower. Not an expert on this.
and if there is a large terrain, this will not cause a performance problem?
Is 20 a lot? Impossible to answer. Do you want an FPS with several bots running around? Or do you want an RTS with 1000 units bumping into each other? Or do you need a multiplayer space-sim and have AI enemies find their way in 3d space using physics?
Blueprints are fine for simple stuff.