Hi @Anu,
Yes as compared to import you will see a significance improvement in the performance. The reasons, from SSAS to Power BI will be a live connection, hence no model will be stored in Power BI. Your analysis will be performed within SSAS (measures etc) and you can further create ad-hoc measures in PowerBI. PowerBI also works on SSAS engine (tabular) for modeling purposes but I like to think of this as subset of full power and potential of an actual SSAS.
A few things that will be different and you will take into account are applying RLS. RLS cannot be applied in the Power BI when the source is SSAS since SSAS have the RLS option. If you are truly using SSAS versus AAS then adding users to the gateway also requires additional steps.
Overall, all my enterprise clients are on SSAS or have migrated to SSAS.
Hope this helps. 
Regards
Ansh