Revit calculations prevents saving, affect performance

Revit has its own mechanical calculations (ventilation and piping). They are background processes which are not visible for user. When Revit performs calculations, other functions may suffer. Software may freeze, you might get different errors and messages about ongoing calculations. During the calculations you might not be able to do anything.

There are some knowledge base articles in Autodesk related to these issues:

https://knowledge.autodesk.com/support/revit/learn-explore/caas/sfdcarticles/sfdcarticles/How-to-disable-background-calculations-in-Revit.html?us_oa=dotcom-us&us_si=5ab36ef2-1f69-444a-b8f6-26a3aaf62502&us_st=piping%20system%20calculating

https://knowledge.autodesk.com/support/revit/troubleshooting/caas/sfdcarticles/sfdcarticles/Revit-fails-to-snychronize-with-central-due-to-system-volume-calculation.html

First you should try is to set all ventilation and piping calculations in Revit Mechanical settings to "Performance". Another option would be "None". This applies when you don't use Revit calculations but you use MagiCAD calculations.