If you run Commander styled setup. You have the Context Menu on RightAndHold on the right button.. Here button need to be pressed for 1 sec.
But you also have it on Double click of right button..Then it is called directly.
For Explorer Styled when ContextMenu is on right button down. then it is called directly.
But depending on what kind of Shell Extension you have installed it can take some time to build the popup menu.
All registered Shell extensions you have installed are going to be called and they do all kind of work to add items to the menu, Every time it is shown.
This is out of the control of MC. And the speed depends on what the shell extensions are doing. MC is not involved in that process at all. Windows handles all the loading and calling of the shell extensions.
ShellEx View is a tool by Nir Sofer where you can list all your shell extension and enable/disable they easily.