The transient database file is a database in its own right, just like the PDB persistent database. There is currently no way to diagnose what is happening with it, for example if there are a lot of IOs to it for no apparent reason, when the applications using it are idle.
It would be helpful if Monitor could show database stats for the transient database files, like it does for the pdb. As there is one per JADE process, it would presumably be in the Process Information group. It would contain info similar to the Database Statistics group, with gets, puts, misses,etc. Some provision would be needed for shared transient overflow.