Apple states that the app using the background mode shouldn't perform expensive, battery consuming tasks when launched in the background. What exactly is considered a batter consuming task? To be specific: is searching and array of 100 entries acceptable? What about 1000?
A battery-consuming task is just that - a task that uses so much CPU that it makes considerable "dent" on the battery, as measured by the "battery percentage" screen. To that end, searching 100, 1000, or 1000000 items a single time is unlikely to do any damage. On the other hand, searching a 10-item list fifty times a second is very likely to make your task a high energy consumer. Same goes for downloading data multiple times per minute, using location services, etc.
A rule of thumb is very simple: go to the "Last 24 hours" view of the "Battery percentage" screen, and see if your app is listed there. If it is not there, of if it is below "Home & Lock Screen", your app is fine.