New UX is more intuitive, easy to use and better way of showing the data. You are right in saying that an app of dashboards can be from multiple models which was not in the case of classic dashboarding but I will disagree to your second point that opening a dashboard within a model is slower than opening an app from New UX. The reason that I am saying it is because even when you open an app in a PC it will have to bring the model back into memory which will take equal amount of time if not more/less.
Great observation on accessing app thru mobile. Personally I haven't experienced any delay in the performance or load times. I can be wrong but that is my personal observation.
It looks better than old and looks more inline with todays style
It is more efficient than old UX . In old UX you can open multiple tabs within anaplan but new ux will not let you . It always opens of one page at a time . This keeps the cloud servers in the background running more efficiently
New UX renders better on modern technology devices like tablets and mobile phones .