Why does Arduino IDE take so long to open?
The simple answer is "libraries".

Every time you start the application, all files inside your library folder are parsed.

And this is not done over your OS, but over the Java-Virtual Machine. 

So it is not just slow, it is ultra-slow! Since your antivirus will probably also check each file.

Once, all files was read (maybe 1/2 of the time spent by your Arduino start), all your sketch folders are parsed.

At the end Windows devices and Arduino specific libraries are loaded.

On my Arduino IDE (is installed locally), on each starts, the Arduino IDE checks about 300.000 files. I need up to 20 seconds to load everything, before I can use the IDE.

What can you do?

Base


do not waste the library folder with unused libraries
NEVER put some documentation or non-Arduino files in your sketch folder, as they are all parsed by the IDE
disable the Antivirus for the folders:


Arduino
libraries
sketch



Advanced

If you open always the same project, you can for example edit the "preferences.txt" file.

If you remove some parameters from the preferences.txt, you will be able to lauch your IDE much faster. I created an application that statrs your Arduino IDE with personalized settings:
https://github.com/Adrianotiger/ArduinoIDE_Launcher  

This application can remove some preferences like:   


boardsmanager  
last.* settings
recent sketches


when the Arduino starts, so it will be launched faster. But if you set the preference sketchbook.path, you can start your Arduino IDE 3-4 times faster. Just place the folder with your project libraries and you can start the IDE much faster.