I have a relatively large model, and it takes about 3 seconds to rotate or move the mouse after pressing any key. How should I optimize it? Can anyone tell me?
short answer: YES!
You have too many faces/vertices. And a lot of meshes !
I don’t know what you meshes look like but there are many way to improve.
Use instancing if you have a lot of duplicated meshes.
If you triangle density is high, you can try to use normal maps.
I guess loading time is high as well.
You should definitely disable picking on your scene 
I just started learning model, don’t know much about how to optimize the grid, I will be all merged grid directly by blender, and then loading speed is satisfactory, but there is a very headache problem, is a memory leak, continuous jump routing, if I reload the memory model will have been soaring, until the crash, I asked this question before community, It has not been solved, I do not know whether it is the model problem or the BabylonJS problem, can you help me to take a look?
Can you please provide a playground so we can investigate?