node.js serverside 2d search performance

I plan to store and update user locations (lat, lon, userid) via websocket on a nodejs server. my goal is to broadcast the user locations to every user, as fast as possible. (like the mytaxi position of each taxi in the app)

my concerns / problems

1. server side performance on lots of simultanious users
2. pushing the data back (i only need to know about users in my region) -> 2d search (get users which lat / lon is in boundingbox )

questions:

1. whats the best storage solution (mongodb vs js array / object storage)
2. is read/write on db faster than array searching?
3. is there a 2d optimized javascript search solution?

my way

i would go for two arrays (arr1 sorted by lat, arr2 sorted by lon) -> search via divide and conquer -> check on similar ids -> output

is there a better way to do this?

thanks in advance

alex