For your first question, the step will run as the same user account that the Octopus Service was set up with. As part of the Octopus installation wizard the person installing it would have been given the opportunity to decide which user the Octopus service runs as. It defaults to the System Account. To check your installed instance, on the machine where Octopus is installed go to Services, right click on ‘OctopusDeploy’ select ‘properties’ and have a look at the ‘Log On’ tab.
The answer to your second question is yes. Running steps directly on Octopus Deploy has an impact on how many tasks can be running. Any step running on the server will take up a processing slot which defaults to 5. There’s a configuration option in Configuration > Nodes, that’s called Task Cap, where you can check if it’s still 5 or has been modified.
If the server running Octopus has enough processing resources you can try increasing the Task Cap and see how you go.