Why you should not uses libraries
1. Most libraries are bloated. The user may load a full library for effect, but in many cases the effect could be achieved in a few lines of code.
2. They unnecessarily abstract the code making debugging difficult. If something goes wrong it could be your code or the library code and it may be hard to determine which. Indeed libraries like prototype.js actually change the language behavior so much that people have likened prototype to crack cocaine.
4. It is really hard to create something really good with universal tool. A standalone script can generally be be optimized much better then library based one.
Aren’t actually faster
But my main problem with libraries are that for complex projects they slow down develpoment.
With ongoing complex projects you are generally extending and enhancing your work continuously. Because of this level of complexity you will generally find yourself having to understand how the entire code base (including the library) actually works. And in programming reconstructing somone elses logic can be more time consuming than actually writing it yourself. This can be even more unfortunate when if you discover there work is flawed and you DO have to write it yourself!