When I tested the performance again, using apache benchmark and also a query profiler I discovered that there was absolutely 0 performance difference between the 2 solutions.
Benchmarks are hard.
What were you testing exactly? The behaviour you are seeing is surprising, but if the database server is on the same physical server as the app server and you are testing for response time without loading the server I could see how it could happen.
Did you have indexes setup to support the joins?
Did you have sufficient RAM for the join to be done in memory?
Was the database server hitting IO limits?