What do the add and delete tables look like? If the users doing "long term editing" aren't reconciling regularly, the add and deletes can get bloated, and compressing the geodatabase won't be able to compress much.
To determine whether it is the workflows of these long term editors that are causing the issue, I suggest you work with them to reconcile and post so you can get a clean compress. If doing so improves the performance, then you know you need to focus on workflow changes.