Having just started using Portfolio I find it incredibly confusing. We have a very small team and so it is unlikely that more than one single team member will ever work on a particular epic. What my Portfolio schedule is assuming is that a team member would be happy to context-switch from one epic to another according to Portfolio's inner algorithm. The result is that all the epics appear to run concurrently but take several years to complete.
Is there a way to treat epics as an atomic unit that must be completed before starting items in another epic?