Local Mongo Server vs ".meteor/local/db"?

My project is very slow when uses login at the same time (local network).
Should I use the Local Mongo Server or “.meteor/local/db”? what difference?
Please help me.

.meteor/local/db works for dev-mode only. It is better to build and deploy your app on a separate server. Also give a try for kadira to find out bottlenecks in your app.

Could you explain/how to use Local Mongo Server on my local machine?

Just install Mongo for your platform. Create a new database. Then say meteor which where your db is placed, define env variable - just type in terminal: export MONGO_URL=mongodb://localhost:27017/{name_of_your_db} - where {name_of_your_db} is the name of DB you created. Start meteor!

Thank for all, I will try.