What is the optimal speed relation of CI/CD jobs to a (reference) local machine?
You want the execution speed of the CI pipeline as fast as (practically) possible, irrespective of being a local or shared setup or the time required to set it up - this is what effectively dictates the overall development speed in the respective project.

But it should be noted that by speed here I don't mean the duration of each CI execution. The duration of such execution may be directly related to the coverage of the CI QA verification, which translates into stabiliity of the development process and the quality of the product. For example including a smoketest in the CI verification will increase the duration of the CI execution, but will also raise the product quality - it's a good thing.

But I have to mention: such private CI setups (even those SLA-backed CI setups you mention) are often an indication of what the ThoughtWorks team called the CI Theatre. Simply using CI tools doesn't make the process a CI one. Using project/feature/integration branches which still require subsequent branch merges (other than cherry-picked fixes) before release means development is still done in silos (those branches are the silos) - waterfall style. 

Side note: Frankly, if I'd be part of a team in which management doesn't understand the importance of CI (let alone not being able to effectively communicate on such critical subject) I'd look for another job - I doubt they'll remain in business for very long ;)