That's exactly how the real world works. Databases will get slow, then slower. Resources get used.
Unpredictable not really. Maybe you've run out of space or ram or processes are hanging. The database will never just start rending html or formatting your disk or email someone. It is pretty predictable.
The failure I've seen multiple times is that the database is returning data within normal latencies, and then there is a traffic tipping point and the latencies go up 1000x for all requests.
