xoconostle, here's what the manual has to say about this option:

"Optimize last render pass: selecting this will optimize the last render pass, making render up to 3 times faster, but leaving out minuscule details on occasion. You cannot generate G-Buffer
information in this mode."

In other words, the render engine "cheats" to make the render faster. In practice, I notice some blur in areas with high detail.

Generally speaking, whenever you see the word "optimize" in Vue, you should suspect. Generally, that word means "less quality". Just search the Vue manual for "optimize" and you'll see that there are almost always some kind of trade-off between quality and speed.

Some times, the optimization doesn't translate in any visible loss of quality (for example, the "optimized" in the dialog box "anti-aliasing options", as oposed to "systematic") and some times it's really important that you select it or you'll be facing impossibly high render times (liike in GR, the option "optimize for infinite scenery"). But generally speaking, you should read "less quality" where you read "optimize".