The reason that you’re seeing a difference is due to the execution plan that the planner is putting together, this is obviously different depending on the query (arguably, it should be optimising the 2 queries to be the same and this may be a bug). This means that the planner thinks it has to work in a particular way to get to the result in each statement.
When you do it within the JOIN, the planner will probably have to select from the table, filter by the “True” part, then join the result sets. I would imagine this is a large table, and therefore a lot of data to look through, and it can’t use the indexes as efficiently.
I suspect that if you do it in a WHERE clause, the planner is choosing a route that is more efficient (ie. either index based, or pre filtered dataset).
You could probably make the join work as fast (if not faster) by adding an index on the two columns (not sure if included columns and multiple column indexes are supported on Postgres yet).
In short, the planner is the problem it is choosing 2 different routes to get to the result sets, and one of those is not as efficient as the other. It’s impossible for us to know what the reasons are without the full table information and the EXPLAIN ANALYZE information.
If you want specifics on why your specific query is doing this, you’ll need to provide more information. However the reason is the planner choosing different routes.
Additional Reading Material:
http://www.postgresql.org/docs/current/static/explicit-joins.html
Just skimmed, seems that the postgres planner doesn’t re-order joins to optimise it. try changing the order of the joins in your statement to see if you then get the same performance… just a thought.