I use https://www.writelatex.com/ where I have a 17 pages (pdf) of math exercices, and I don't know how they deal with it, but apparently they manage to make incremental "recompilation process", because when I make an online modification on the Latex code, the modification appears almost anstantaneously on the preview pdf aside the code !

You are lucky. I am working on a ~80 page report and it takes 40 seconds for a new word to appear.

I'm astonished by that time number. I have a 400 page book, with a 150 page answers to exercises that is emmitted as part of the book compilation and then compiled on its own. I use a simple shell script that compiles each twice (for the cross-referencing). It takes perhaps 10 secs on my five year old middle of the road laptop. Are you remaking complex drawings each time? What is the performance sink, I wonder?

Sorry, it seems like there was a misunderstanding. It takes this on writelatex.com, not my local machine. There it's a normal 7-8 seconds. Just text and about 50 png/jpg images.

My workaround to cut down compilation time is splitting up the document into parts and always only compiling the part I'm working on.

I have tried a similar work around but it turns a bit messy if you are in the paper publication business (which I am). Camera ready copies (final publication) sucks the remaining "work around motivation" out of you.

In case people don't know, there's a built-in command to do this: \includeonly.

I am not sure if they are doing an "incremental" process. From what I understand, they have a real time preview which compiles the document in the background.