[mjs]

This is also pertinent:

http://www.mongodb.org/display/DOCS/How+does+concurrency+wor...

Note that whilst a write blocks all other writes, the time you need to wait is the time it takes for the in-memory data structure to be modified; you don't have to wait for the write to hit disk. (Writes are only persisted to disk every so often.)

[rb2k_]

Unless you actually don't have enough RAM and that data structure has to be paged in... that's a painfully long wait