We are using Universal WebViewer, and currently we are using e_off now for creating XOD files. What will be the difference switching to e_fast?
When you switch to e_fast, some (non-glyph) vector content will be converted into an image. So at some point zooming in, depending on the DPI setting, your users will see pixelation.
Essentially with fast mode, some graphically intense content (typically complex patterns) will be converted to an image server side, therefore sparing the client from having to do these complex drawing at runtime. Especially useful in cases where the client device as far less CPU speed than the server.
A good rule of thumb is to take the maximum zoom that your UI allows, say 400%, and multiply that by 72 (native PDF DPI). So on this example, the DPI would be 4*72=288. This way your users will probably not be able to tell the difference. You can also switch to PNG mode (SetPReferJPG(false)) to avoid compression artifacts common to JPG.
The downsides to e_fast are loss of vector data, increased file size, and if using JPEG, compression artifacts.