you are right to bring up speed as a factor. one interesting study found subway escalators are more efficient when people stand close compared to walking up! Space trumps speed in this case.

In urban environments, cars have a low average speed (e.g. in London UK it's around 14mph) so I think space will dominate here too.

Also remember that as speed increases, so does spacing for safety reasons.

Cycle infrastructure should be built by taking space from cars, not busses.