First, I would use the EXPLAIN tool to look at the access paths of both queries. This will allow you to see that it is accessing what you think should be the correct indexes, etc.
Then to confirm, set up a STATEMENT Event Monitor. Start it, then run your two queries. You will need to do this a few times and change the order of running the queries as the bufferpools may mask some inefficiencies. After each run, you need to analyze the output of the event monitor.
There is a performance monitoring tool from Embarcadero called DBOptimizer.
Form their site I found: Embarcadero DB Optimizer maximizes database and application performance by enabling DBAs and developers to quickly discover, diagnose, and optimize poor-performing SQL. DB Optimizer eliminates performance bottlenecks by identifying data intensive or frequently executed queries, focusing on specific SQL statements through query statistics (CPU, I/O, wait times), and fine-tuning problematic statements.