will it be optimized if i shorten the layer like this?

After Effects calculates every pixel on every frame. If you set an out point for a layer, those pixels are no longer included in the calculations.

Just to add to Mylenium's comments - I think if you are concerned about render times, then look at any common elements or render-heavy elements that you can render out separately with all the effects on.  This will mean your not slowing down your overall render, but still able to tweak everything else.  It might be worth planning your render time to be when you're away from your computer too - I might finish a project in the morning, but I know if I set my computer rendering, it could be out of action for a big chunk of the day.  So, I'll move onto the next project and set the original rendering when I'm done for the day.  It annoys my wife as I keep getting up to check the render status, but you can't have everything.

Long and short: No. Since AE's render order is stricktly based on teh stacking order and the properties, it is kind of self-optimizing anyway. Yes, you may be helping mnemory consumption by not loading soem content in certain sections and you may also prevent effects from screwing up section where they aren't even doing anything, but otherwise you don't really gain anything. trimming the layers just in the hopes of performance gains really does not do much.

Mylenium