You could run mysql with the memory storage engine and with our schema--which would require multiple outer joins OR subqueries--mongodb will still be much faster. So I think it's much more than an fsync issue.
MySQL's deficiencies aren't inherent to SQL databases. Other databases have faster query parsers and better query planners. It seems that with all the time and money invested into NoSQL solutions, Postgres could be improved to the level of Oracle or DB2.