Overhead of a Round-trip to MySql?
There is always overhead in database calls, in your case the overhead is not that bad because the application and database are on the same machine so there is no network latency but there is still a significant cost.
When you make a request to the database it has to prepare to service that request by doing a number of things including: 

Allocating resources (memory buffers, temp tables etc) to the database server connection/thread that will handle the request, 
De-serializing the sql and parameters (this is necessary even on one machine as this is an inter-process request unless you are using an embeded database)
Checking whether the query exists in the query cache if not optimise it and put it in the cache.


Note also that if your queries are not parametrised (that is the values are not separated from the SQL) this may result in cache misses for statements that should be the same meaning that each request results in the query being analysed and optimized each time.

Process the query.
Prepare and return the results to the client.

This is just an overview of the kinds of things the most database management systems do to process an SQL request. You incur this overhead 500 times even if the the query itself runs relatively quickly. Bottom line database interactions even to local database are not as cheap as you might expect.
So I've been building django applications for a while now, and drinking the cool-aid and all: only using the ORM and never writing custom SQL.
The main page of the site (the primary interface where users will spend 80% - 90% of their time) was getting slow once you have a large amount of user specific content (ie photos, friends, other data, etc)
So I popped in the sql logger (was pre-installed with pinax, I just enabled it in the settings) and imagine my surprise when it reported over 500 database queries!! With hand coded sql I hardly ever ran more than 50 on the most complex pages.
In hindsight it's not all together surprising, but it seems that this can't be good.
...even if only a dozen or so of the queries take 1ms+
So I'm wondering, how much overhead is there on a round trip to mysql? django and mysql are running on the same server so there shouldn't be any networking related overhead.