Disable this option under Debug->Options->Suppress JIT optimization on module load (Managed only).
A connect report submitted before:
https://connect.microsoft.com/VisualStudio/feedback/details/2116788/flag-optimize-is-passed-to-the-debugger-even-while-the-build-settings-optimize-code-is-not-enabled-on-mvc-c-web-projects-when-using-just-my-code.
Please also make sure that all projects are in debug mode, Optimize code was unchecked like this case:
VS2015 Project no longer runs in debug mode