Our repo has about 160k objects with 1.6M datastreams. With breadcrumbs enabled, objects load between ~12-20 seconds. With them disabled, objects load between 3-5 seconds.
Breadcrumbs are an integral part of the user experience, as is page load times. Using a drupal contrib breadcrumb module isn't viable as islandora doesn't represent inheritance in a way that is visible to drupal.