Improve Perfomance in Delta Mode

# Feature request

## Description

When delta mode is enabled and only when two maps are selected, I want the new delta map to be calculated.
Currently, the map is re-rendered/calculated every time delta mode is enabled and no or one map is selected.

## Acceptance criteria
- Only when two maps are selected in delta mode the map has to be recalculated/rendered.

## Development notes (optional Task Breakdown)
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Start by tracing where delta mode responds to map selection changes and where the map is recalculated or rendered. Verify the behavior with no map, one map, and two maps selected; done means recalculation and rendering occur only in the two-map case.

Written by the indexing model from the issue text.