Will using plugins cause any performance issue?

While developing a game, Will using too much Plugins cause any performance related issue?Is there any side effect on size of ".exe", ".apk" etc?

 

That would totally depend on the plugins used. Some are editor-only, some do stuff during runtime, some do stuff only if you ask them to. The question is too broad to give a single answer.

 

The best way to look if it's causing performance issues is to first implement your plug-in, then use the Unity Profiler, which tells you what is giving you performance hits.

 

Plugins that are in the Editor/ folder (or checked as editor-only in the import settings of Unity 5) are only imported and loaded in the editor, so they won't have an impact on the build size or the build's performance.

Regarding performance for other plugins, well, as blizzy said, it totally depends on what the plugins are doing.

 

@Will H and @blizzy
Thank You for your information.
Ok Let me more specific.
1. When I Import any plugin into Editor, there are many Items i.e. Scripts, Animations, Prefabs, Materials . Will the unused items be included in the build package? or Unity Checks the usage.
2. For example, I am using iTween for animation, Photon for Networking, Plugins for different social community i.e. Facebook, G+ etc. And many things I can do manually, but it has been done in different plugins. Now Using those plugins will save my development time but what about efficiency?

 

Unity will only include what is referenced in your project. It will, however, include all code. That should be the least of your concerns, though, since code tends to be small.