UseGDIPlus is used to use GDIPlus canvas drawing. The GDIPlus canvas supports transparency along with antialiasing that GDI does not. So for the highest quality canvas drawing use GDIPlus. One disadvantage of GDIPlus is that the drawing methods are slower than GDI.
As far as I know, the only thing that increases the ability to quickly handle large images is by using the IELib dll's. The dll's increase loading speed by about 25%. Generally when speed and increased memory is needed by my projects, I use the dll's and compile as Win64.