How fast is it to look up by ObjectId in Mongodb?
A more precise answer: MongoDB uses B-Tree indexes. Searching for a particular value in a B-Tree has O(log n) complexity in the average and worst case, which can be considered reasonably fast (i.e. a binary search). It is not constant complexity = O(1) though, so you still might have some slowdown effects if the index size grows larger than available RAM. (MongoDB tries to keep the indexes in RAM, and every IO needed to look up an index on disk will slow down your query considerably).