Improve performance of RuboCop and its cops

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Problem

The rubocop job on CI takes currently about an hour to complete. As we are enabling more and more 👮 cops it will get worse over time.

Proposed solution

Improve performance of RuboCop:

1. Profile RuboCop itself and used cops to find hot spots
2. Provide patches to improve speed
3. Upstream these patches to RuboCop or gitlab-styles