> For the parameters min_size, max_size of 5 and 30 it takes over 2 minutes
> and for 1 and 10 it takes about 25 secs. Any idea why this is so long?

In general it is strange. Obtaining a connection to Firebird takes
milliseconds, seconds at most. The test suite for Jaybird is executed
less than in 6 minutes and it creates database, obtains connections,
executes tests, closes the connection and drops the database about 500
times.

> Here's the code I am using,
>

...

Do you close your connection that you have obtained using the
Connection.close() method?

In general try logging on the INFO level - you would see getting the
connections from the pool:

http://jaybirdwiki.firebirdsql.org/jaybird/doku.php?id=config:logging

Roman