I understand that you have updated MATLAB from version R2017b to R2021b, now that you are trying to test your old simulations on the new update, they seem to work well in the “Rapid Accelerator” mode but gets stuck in “Normal” and “Accelerator” mode, which was not seen in R2017b.
Please know that the lag and hanging of Simulink usually happens because of the time steps taken by any solver becoming extremely small.
You can refer to the following MATLAB Answers thread where a similar issue has been addressed:
However, it is possible that the updated version of MATLAB might have deprecated certain functionalities that were present before or replaced with new functionalities. Redesigning the model or replacement of the blocks that have been updated in the new version after referring to the release notes might help.
Please refer to the following link for MATLAB Release Notes:
Hope this helps.
