concurrency exercise (oracle sql)

It takes a long time to run (the point of the exercise is to observe that it takes a while to run). As instructed in the exercise: I cancel the query.

The question: why did the query take so long to run? (before I canceled it)

Context: I've run a number of other select, update statements within the last 5-8 minutes testing concurrency problems. The exercise appears in a chapter on concurrency problems in SQL.

My theories are slow server, slow network, bottleneck. I don't have any theories that cite concurrency specifically. Please look over my question and add if possible to my list of reasons why a sql query takes a while to run.