Hello Xaymar,
This is not a bug, but expected that your computer is going to run a bit slower when shaders are compiling. If you are opening a copied project or converted project for the first time, then you will generally want to wait for your shaders to compile as your machine is utilizing a good bit of processing power to compute the shaders. The 283 materials and 14,000 shaders sounds about the right number. Shader compile times depend on the complexity of your materials and your machine.
Within each material you can toggle the statistics to view the instruction count for static and dynamic lighting, vertex shader, and texture samplers. You can also use the Shader Complexity visualizer within the viewport toolbar to observe the complexity of your materials placed in the scene. I assisted a user not long ago who had a question about optimizing his shaders.
If you have further questions or need additional assistance, please let me know.
Thank you,