This cross-platform issue reproduces on all IL2CPP capable platforms, including 'PC, Mac & Linux Standalone'.

Performance of code is very poor if using IL2CPP with the 'Script Debugging' option enabled, even if you do not connect the debugger.

Steps to Reproduce
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1. Download the users reproduction project, project is already set to use .NET 4.x and IL2CPP
2. Open the 'TestScript.cs' script file, change line 27 from 'Debug.Log' to 'Debug.LogError' so the 'time to execute' is output to the screen.
3. In 'Build Settings' switch platform to 'PC, Mac & Windows Standalone'.
4. Turn off 'Script Debugging', leave 'Development Build' enabled.
5. Build for Windows Standalone
6. Run the game

Result #1 - without script debugging
The 'TestScript.cs' script takes ~0.26 seconds (260.7765ms) to complete.

7. Enable 'Script Debugging' in the build settings
8. Build for Windows Standalone
9. Run the game

Result #2 - with script debugging
The 'TestScript.cs' script takes ~19 seconds (19873.11ms) to complete.

Reproduced in
2018.2.15f1 (65e0713a5949)
2018.3.0b9 (a3326ae325b2)
2019.1.0a6 (370df4c90d4b)