Ups...
"BAS is better performer than MDX" - what do you mean by BAS in this case 
"BPC uses to calculate child nodes with SUM, AVG" - BPC is not using AVG to calculate nodes, aggregation is alway done as SUM and standard measure formulas use some other function, but not AVG.
There is no big difference between complex and simple KPI's, both are slow . You have to test always to see if the particular report performance is acceptable or not. Sometimes to calculate KPI you will need to create extra members calculated with script or badi like here: Average percent values on nodes
And yes, Excel formulas is always an option
Vadim