The benchmark you're reading is probably wrong
Mikeal Rogers wrote a blog post on MongoDB performance and durability. In one of the sections, he writes about the request/response model, and makes the following statement:
MongoDB, by default, doesn't actually have a response for writes.
In response, one of 10gen employees (the company behind MongoDB) made the following comment on Hacker News:
We did this to make MongoDB look good in stupid benchmarks.
The benchmark in question shows a single graph, which demonstrates that MongoDB is 27 times faster than CouchDB on inserting one million rows. At the first glance, the benchmark immediately looks silly if you've ever done serious benchmarking before. CouchDB people are smart, inserting such a small number of elements is a relatively simple feature, and it's almost certain that either they would have fixed something that simple or they had a very good reason not to (in which case the benchmark is likely measuring apples and oranges).
Let's do some back of the envelope math. Roundtrip latency on a commodity network for a small packet can range from 0.2ms to 0.8ms. A single rotational drive can do 15000RPM / 60sec = 250 operations per second (resulting in close to 5ms latency in practice), and a single Intel X25-m SSD drive can do about 7000 write operations per second (resulting in close to 0.15ms latency).
The benchmark demonstrates that CouchDB takes an average of 0.5ms per document to insert one million documents, while MongoDB does the same in 0.01ms. Clearly the rotational drives are too slow to play a part in the measurement, and the SSD drives are probably too fast to matter for CouchDB and too slow to matter for MongoDB. However, CouchDB appears to be awfully close to commonly encountered network latencies, while MongoDB inserts each document 50 times faster than commodity network latency.
At first observation, it appears likely that the CouchDB client library is configured to wait for the socket to receive a response from the database server before sending the next insert, while the MongoDB client is configured to continue sending insert requests without waiting for a response. If this is true, the benchmark compares apples and oranges and tells you absolutely nothing about which database engine is actually faster at inserting elements. It doesn't measure how fast each engine handles insertion when the dataset fits into memory, when the dataset spills onto disk, or when there are multiple concurrent clients (which is a whole different can of worms). It doesn't even begin to address the more subtle issues of whether the potential bottlenecks for each database might reside in the virtual memory configuration, or the file system, or the operating system I/O scheduler, or some other part of the stack, because each database uses each one of these components slightly differently. What the benchmark likely measures is something that is never mentioned - the latency of the network stack for CouchDB, and something entirely unrelated for MongoDB.
Unfortunately most benchmarks published online have similar crucial flaws in the methodology, and since many people make decisions based on this information, software vendors are forced to modify the default configuration of their products to look good on these benchmarks. There is no easy solution - performing proper benchmarks is very error-prone, time consuming work. It's good to be very skeptical about benchmarks that show a large performance difference but don't carefully discuss the methodology and potential pitfalls. As Brad Pitt's character says at the end of Inglourious Basterds "Long story short, we hear a story too good to be true, it ain't".