When object history entries are created, they are signed as explained in Chained signature for object history in Identity Manager. Many users may use Identity Manager in parallel, but a chained signature requires a strict ordering of the entries. Furthermore, as the signature of the previous entry is also signed by the next entry's signature, entries need to be signed sequentially. This introduces a bottleneck, as users access the system in parallel but the history they generate must be signed sequentially.
When history entries need to be signed, they must wait in a queue for any prior entries that have not been signed yet. When new entries are generated faster than they can be signed, this can effectively block the system. A growing queue is a telltale sign that this situation may be approaching.