As with any performance considerations the main item of concern is:
As such JoSQL tries to do the minimum amount of work it can do to get the job done. Since JoSQL is built on the Java-reflection architecture it should only really be used in Java 1.4 and upwards since before that time the reflection performance was fairly poor.

In testing JoSQL seems to scale fairly well, with non-linear performance being observed. Details about timings and queries coming soon.

In general, the more work you expect JoSQL to do the longer it will take to do it. If your WHERE clause does not significantly limit the number of objects that the rest of the query should work on, then obviously the rest of the query has more work to do, which equates to more processing time.