Background re-indexing vs Foreground re-indexing

Hi everyone. We have a customer who is seeing really huge differences in the time it takes to do a full re-index, depending on whether they elect to perform the re-index in the foreground or the background. They’ve contacted us because they see the most pronounced difference when our app is installed in their instance. Can you think of any reason why an app could cause an excessively slow re-index when in background mode, but not in foreground mode?

Thanks heaps for your attention.