The first 30 minutes of the fortnightly Diagnostic establish your baseline across subtests. The final segment — sustained attention under fatigue — is where the endurance measurement happens.
I have sat four fortnightly Diagnostics. In every one, my per-item accuracy drops measurably after minute 30. The case report graphs this as the "fatigue slope" and it is the single largest drag on my composite.
What helped: Recital-length practice sessions (45+ minutes) twice per week. Not for score improvement, but for fatigue tolerance. The goal is not to be sharp at minute 35 — the goal is to be less degraded at minute 35 than you were last month.
The Society measures endurance because endurance is the faculty being eroded. Train the measurement.