I will add:
Box - restrictions are imposed only by the server on which you install OneBox. The server is your choice.
Cloud - restrictions are imposed by the cloud servers on which OneBox is installed. The allocated resources strongly depend on the tariff.
Since OneBox is mostly a constructor, everything depends heavily on the combination of actions and procedures that are hung on the stages.
OneBox is designed so that for a server like https://www.hetzner.com/dedicated-rootserver/ax101
you can easily upload 100M contacts and 100M orders. And it will bring down the API at 5000 orders per second.
But, if the action "knock somewhere in the external API" is added to the insertion of an order, the speed will drop dramatically.
If you add 50 different actions to "calculate prices" - the speed will drop too.
If there are no actions, there will be thousands of processing per second, if not tens of thousands.
The search is almost everywhere used by fulltext, there by the complexity of Log (N).
That is, that there are 1 million orders in the search, that 10M - the difference for the search is not significant.
Better give a specific case of what and how you want to use.