The default installation of SSMS has a launch sequence of:
This third step may be what’s slowing down your start-up. You can change the default to skip the third step.
To do so, expand the Tools menu and choose the options item (down at the bottom). This opens the Options window.
Find the “Startup” section in the left window and use the drop-down list to change the default to “Open empty environment”. This picture illustrates the setup: