See this post By Oleg Troyansky:
Optimized load is much faster and therefore preferable, especially for larger data sets.
Optimized loads are possible if no transformations (calculations) are made during the load, and no (almost) filtering is done in a WHERE clause.
The only acceptable filter is using function Exists:
So for example this load can't be optimized:
Load * From ... WHERE Country = 'US'
but this load can:
Load * From ... WHERE EXISTS(Country)
On the other hand, the only time you want the load not to be optimized, is if you load a Mapping table from a QVD file. In this case, Optimized load doesn't quite work, so you want to make the load non-optimized deliberately.