I'm developing a web conference application . Is is for the best of time and performance to use an existing open source webconference and ad some fonctionnalities or start it from scratch ?
Thanks in advance
That really depends on what you want your application to do.
E.g. Whether existing opensource applications / libraries meet the requirements for your application.
As you haven't specified what time or performance constrains or other requirements for your application are it is difficult to make a recommendation due to lack of information.
If you do decide to use existing Open Source libraries or software, I suggest you have a look at the following projects, even if it is just to give yourself a better idea of your application's requirements.