Available power may be shared according to the hob's design and installation setting. The total active load matters, not just one zone's selected number.
What the observation means
Different water volumes and pans can also confuse the comparison. A shared-power explanation should come from the manual rather than being assumed for every slowdown.
A useful next step
Record the active zones and settings during ordinary use. Do not change installation power limits yourself or run repeated maximum-load tests. If the observed behaviour conflicts with the specified arrangement, provide the model and display information for assessment.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.