How does node-red allocate system resources, like CPU and RAM across flows? Are there any benefits/downsides to putting all nodes on one flow vs splitting them up on multiple flows in terms of resource allocation? For instance, will a dashboard be more responsive if all the nodes for that dashboard are on one flow or split up between multiple flows? Does it even matter?
Node-RED doesn't do any resource allocation as you describe.
There are certain things that are scoped to the flow - such as flow context, Catch and Status nodes. So there are functionality reasons to choose to split your flows amongst multiple tabs. There are also reasons around basic management and organisation of your flows.
But there will be no benefits to the dashboard by putting all your nodes on the same tab.